when donna and walter bernadzikowski met at a church social, talk of a family even preceded the engagement ring. >> he used to scare the girls away by saying he wanted 12 children. and i said, well, i always thought i wanted to have 12 children. he says, oh, my god. >> reporter: they didn't have 12, but this devout roman catholic couple did raise their five in a baltimore, maryland suburb. heidi, the only girl in a sea of brothers. >> when i hear that, i think poor heidi. >> yep, yep. >> reporter: growing up with all those boys. >> well, and we had friends that used to say to us, oh, she must be treated like a princess. and i said, are you kidding me? she had to be just about tough as they were. >> and she kept them in their place too, i mean. >> she wasn't treated like she was a little princess that's for sure. >> reporter: heidi grew up to be a happy, outgoing, athletic girl, surrounded by friends. it was a noisy, loving family with strict rules. everyone at the table for dinner. mass on sunday and no back talk. heidi's brothers frank and harold. >> reporter: true that your mom was known to wash out a potty mouth with a bar of soap? or was that a metaphor? >> i got it. >> true. >> reporter: did you? >> true. >> yeah. they were very strict. definitely had a bar of soap from time to time. >> reporter: as she grew into her late teens heidi started to bristle under her parent's strict house rules. by 19, she, the next to youngest, became the first to leave the family nest. but the grass wasn't greener. right away, reality offered up a dead-beat roommate; and a succession of low-paying jobs that evaporated like the morning dew. and yet she could always count on her girlfriends to buoy her up. and it was one night with her bffs shooting pool she caught the eye of a guy holding a cue stick. it was stephen cooke. >> they were shooting against one another. and he noticed heidi in the bar. >> reporter: kim is stephen cooke's sister. >> the first night they actually went home together. and three days later, they moved in together. >> reporter: fast. >> so it's pretty -- >> reporter: fast romance. >> it was really fast. really fast. >> reporter: stephen was five years older than heidi. his sister says he could be quiet and shy but always wanted to be around heidi. >> they did everything together. i don't think they that they really did much without each other. >> reporter: in 1998, stephen and heidi moved to this rental townhouse in dundalk, maryland. stephen's dad steve sr. says after living together for nearly two years, the couple was talking about taking the next step. >> was it your understanding steve, they were going to get hitched? >> yes, in vegas and every time i talked to them that was the their plans. it never changed. he loved her. >> reporter: the timing seemed good. heidi had just gotten a promotion at the insurance company where she worked. she was finally a 9:00 to 5:00er after years of temping. >> she was just getting started or getting settled into a niche when things happened. >> reporter: strange things that rattled heidi. in april of that year, it appeared someone was trying to break into their townhouse. there was chipping around a lock on their basement door. >> looked like the doors had been monkeyed with? >> yes. >> reporter: not long after that discovery of mischief on the locks, a stranger knocked at the front door one night saying he was forming a neighborhood block watch. he scared heidi so much that she told friends. and described the stranger as african-american with a tattoo. stephen demanded new locks and keys from the townhouse management. they were installed on april 19th, 2000. the very next evening was when it happened. >> are you sending somebody? >> yes, i am. >> just send somebody. >> i'm going to. just stay with me okay? >> reporter: an officer dispatched to their home saw heidi on the living room floor. her boyfriend leaned up against the wall, cradling her, and crying uncontrollably. it appeared she'd been strangled, her throat so severely slashed her blood had dripped through into the basement. and there was something else something manson-esque, scrawled on the wall in red lipstick above heidi's body was the number one. >> reporter: when we come back a was a killing keeping track of his victims? >> she's number one and get ready because here comes trouble. >> exactly. >> and a possible suspect. >> i'm thinking maybe this is our guy. butcher. heidi's killed with a knife. ♪ sorry about the holdup, folks. we have some congestion on the runway and i'm being told it'll be another 15, maybe 20 minutes, and we will have you on your way. ♪ runway models on the runway? surprising. what's not surprising? how much money evan saved by switching to geico. i would not wear that lace. hmm, i don't know? fifteen minutes could save you fifteen percent or more. you get to do the dishes.ed... bring 'em on. dawn ultra has 3 times more grease-cleaning power. a drop of dawn and grease is gone. so find a venus smooth that contours to curves, the smoother the skin, the more comfortable you are in it. flexes for comfort, and has a disposable made for you. skin smoothing venus razors. >> reporter: a female police officer was the first on the awful scene. >> she's seeing stephen cooke, holding heidi up against the wall. he's got her wrapped in his arms. >> reporter: stephen cooke, heidi bernadzikowski's live-in boyfriend of nearly two years, told the arriving officer that he came home that night only to find an apparently lifeless heidi in their living room. nevertheless, he tried cpr. at 8:58 p.m., he'd called 911. >> are you sending somebody? >> yes, sir, they're on their way, sir, okay? i want you to stay with me. >> no, no, i can't stay with you. i gotta go with her. >> reporter: baltimore county homicide detectives al meyer and gary childs arrived later that night. it appeared to them heidi had been strangled. it was obvious her throat had been deeply cut with something sharp-edged, probably a knife. >> when you get to a scene like that, detective, does it speak to you? does it explain itself at all what happened here? >> well, it was kind of an odd scene. the living room of the house was no real furniture to speak of. up on the wall to the left of where heidi's body was, was a number one written on the wall. so found that obviously to be odd. >> she's number one and get ready because -- >> right, yeah. >> -- here comes trouble, huh? >> exactly. >> reporter: those first few hours left investigators wondering was this the first signature of a budding serial killer? was there going to be a number two? or was it something else? >> the house was tossed, yeah, ransacked. you've got a potential motive of burglary. and you've got the other motive on the wall of a serial murderer. >> reporter: 20 miles away, walter and donna bernadzikowski got a knock on the door at their severna park, maryland home. it was the police. >> and he says, you have a daughter heidi and, yes. you know, and i said, you know, what's the matter? you know, she's been in an accident? what -- you know, what's going on? and he said, heidi's dead. >> it -- it was quite a shock, and i just started like pinching myself. i've got to wake up from this dream. this has gotta be a dream. >> i think our whole world just changed. >> reporter: a short time later, stephen's sister, kim, got a knock too. >> i remember running into the bathroom and getting sick. and i was kind of -- i couldn't -- i couldn't do anything. >> reporter: heidi's boyfriend stephen, meanwhile, had been down at a police station the whole time being interviewed. officers snapped this picture. his clothes bloodied he said from trying to perform cpr and then cradling heidi's bleeding body. >> now, how much did you know about him or the victim by that point? >> not really a lot. we just knew they were in a relationship. the relationship was good. >> reporter: husbands and boyfriends are always persons of interest, but homicide investigators say stephen was cooperative. he told them how after his shift ended at a local lowe's, he picked up heidi from her job in the car they shared, a 1994 red honda civic. it was 5:45 pm, he thought, when he dropped her off at their home. it was, he said, the last time he saw her alive. the detectives put together a timeline for the boyfriend who'd run a bunch of errands after dropping heidi off. a stop at the atm, followed by a haircut, an oil change at a jiffy lube and then a swing by home depot for a plumbing piece needed to repair his sister's sink that night. he had time stamped receipts for virtually everything and turned them over to the police. >> he's got a very solid alibi for his whereabouts from the time he left work until he picks up the phone and calls 911? >> he does. but you still have to take into account, you know, it doesn't take long to do what had been done. you know, she's strangled and her throat is slit. you know, that could happen in a matter of seconds. >> reporter: after about six hours of police questioning, stephen went home. his sister remembers him being a wreck. >> i could certainly tell my brother was just a mess. he was devastated. i could see in his face that he was -- just looking attitude -- at him he had been crying. >> reporter: in the days that followed, al meyer and other investigators felt a serial killer unlikely the robbery gone bad theory too because there were no signs of a forced entry. but dna was found under heidi's fingernals, presumably from her killer. their hopes were soon dashed however when the sample turned out to be virtually useless. detective gary childs. >> you had a mixture of dna from the victim and you had a mixture of the dna from the suspect. and the technology in those times was that you couldn't separate it. >> reporter: but detective meyer did have one suspect he was very interested in finding. that suspicious neighborhood block watch person. the dark skinned man with a tattoo who'd scared heidi at her front door. and one person of interest was a local butcher. >> reporter: terry gilliam worked with stephen's sister at a safeway just miles from heidi's house. >> i'm thinking, okay. he's african american. i'm thinking, maybe, you know, maybe this is our guy. butcher, heidi's killed with a knife. >> reporter: investigators theorized a connection through stephen's sister. >> they're looking for somebody that comes to the door, did you ever go to heidi's door and knock on the door? >> no. >> saying you're from the block watch association? >> no. actually, i don't even know where heidi lived at. no, never knocked on her door. >> reporter: but the cops wouldn't just accept the butcher's denials and be done with him. there were other reasons to dig deeper, they thought, including an irregularity on his work time card. a time was changed on one specific day. >> it's the day of the murder, one of his times is handwritten in. so the -- >> so the only day of the month? everything else matches. >> exactly. >> now i have him not being able to account for himself the day of the murder. >> reporter: and there was something else -- heidi herself had said the suspicious block watch person had a tattoo on his left arm. >> do you have a tattoo on your arm? >> yes, i do. >> on your left arm? >> yes. >> terry, let me ask you now did you kill heidi? >> definitely not. >> reporter: officials weren't done yet with the butcher and heidi's boyfriend, stephen cooke, remains a suspect too despite his receipt-heavy alibi. >> a lot of time starts to go by. doesn't it? >> too much. >> reporter: at first months go by. then years. more than a decade. no arrests. but there would be one advantage to the passage of time -- breakthroughs in dna technology. and that finally gave police a suspect. who it was shocked everyone. >> reporter: coming up -- it's always the husband or the boyfriend, right? but that didn't seem to be where this investigation was going. >> the headline here is stephen cooke is not the guy. >> right. >> when "dateline" continues. i love you, couch. you give us comfort. and we give you bare feet... ...backsweat and gordo's everything. i love you, but sometimes you stink. ♪ new febreze fabric refresher with odorclear technology... ...cleans away odors like never before. because the things you love the most can stink. and plug in febreze to keep your whole room fresh for up... ...to 45 days. breathe happy with new febreze. with tampax pearl. you get ultimate protection on your heaviest days and smooth removal for your lightest. tampax pearl and pocket pearl for on the go. theseare heading back home.y oil thanks to dawn, rescue workers only trust dawn, because it's tough on grease yet gentle. i am home, i am home, i am home why do people put why does your tummy go "grumbily, grumbily, grumbily"? no more questions for you! ouph, that milk in your cereal was messing with you, wasn't it? try lactaid, it's real milk without that annoying lactose. good, right? -mmm, yeah. lactaid. the milk that doesn't mess with you. ...pampers, stay up to.... ...three times drier, so babies can sleep soundly all night. pampers. >> reporter: as the years rolled by, the heidi bernadzikowski murder case got colder and colder. detective al meyer was frustrated. he was promoted out of homicide but he never forgot the case. >> even when he was in another unit, he would come back up to the homicide unit and go through the file. >> never got squirreled away, huh? >> no, no. never. >> yeah, he never -- he never let it go. >> reporter: heidi's grieving family was trying to get on with their lives, remembering her on her birthday by eating her favorite shrimp alfredo, but family events were hardly the same. >> you couldn't fully enjoy these special occasions because you're always painfully aware that -- >> you had an empty chair. >> you got an empty chair, that she should be there and she's not there. >> reporter: heidi's boyfriend stephen cooke was trying to move on with his life as well. he married, had a child, and landed a steady job with veterans affairs. >> he's got a normal life for the first time in a long while, huh? >> he does. things are looking pretty good for him. >> reporter: then, in 2011, 11 years after heidi's murder, meyer rejoined the homicide unit and once again cracked the file. this time, he and veteran baltimore county detective gary childs got an idea. >> dna technology had progressed and we knew now that there's a possibility that heidi's fingernails may contain some physical evidence. >> so what's the thought, let's run it again, see what happens? >> yeah. the thought was to resubmit. >> reporter: to their surprise, the criminal database spit out a match. >> we got a dna hit. i couldn't believe it. >> reporter: but the hit wasn't for anyone in heidi's known circle, even in her geography. it was a name completely off the radar from a state over 1500 miles away. >> they tell me it's this guy, alexander bennett, from colorado. >> colorado? >> and i'm, like, wow, that's not good. i'm hoping it's going to be somebody from baltimore, somebody that would be local. >> did that name mean anything in heidi's circle, alexander bennett from colorado? >> never heard of him before. >> absolutely not. >> no. >> reporter: baffled detectives started to dig. they called the colorado authorities and learned bennett was an unlikely suspected killer. in his early years, he showed promise as an opera singer. performing recitals and winning a scholarship to the prestigious manhattan school of music. after moving back to colorado though, he'd gotten into some small-time trouble. but then he did something just plain crazy. >> he's a pretty talented guy, but he had some issues with the people he hung with and one of these issues was with a friend of his named grant lewis. >> reporter: in 2003, bennett and grant lewis had been arrested in a doozy of a scheme. they'd called 911 and said that a friend of bennett's wanted to bomb the courthouse. but they went further, building a real bomb and planting it in his house. the bomb squad was dispatched and that friend hauled down to the station for an interview. >> they're grilling him pretty hard because that's kind of a serious crime. one of the detectives ultimately lets him listen to the 911 call and he recognizes grant lewis' voice. >> so it's not all muffled or disguised or -- >> no. no, it's just -- >> he said, that's -- >> -- that's grant lewis, i know him. >> yes. >> reporter: within days, lewis and bennett confessed to the whole thing. building the bomb, breaking into the house, even uploading bomb-related materials to the buddy's computer to ensure he'd be arrested. also that friend who bennett said beat him up wouldn't notice that they'd made off with his jeep. >> so it's all a hoax, this elaborate caper to plant an explosive device in order to get him out of the house so they can steal the car? >> yes. >> my word is harebrained. what's yours? >> yeah. double harebrained. >> reporter: alexander bennett was sent to prison and required to give dna. now, years later, that dna was tying him to heidi's murder back east in maryland. for baltimore prosecutors garrett glennon and matt breault, the dna was an enticing lead but far from definitive proof. >> it was enough to say it looks like it came from him, you know? but we can't say it's definitively his. so there was more investigation to do. >> reporter: so, the detectives went to work, looking for another connection between colorado native alexander bennett and the maryland murder. all the usual computer searches failed, but when sergeant meyer had the maryland state police mine an offline database -- >> i get this phone call from the trooper and he tells me, i've got alexander bennett, somebody running a wanted check on him march 30th of 2000. >> a maryland officer. >> in maryland. i'm thinking, holy cow. >> reporter: three weeks before heidi's murder, an officer had spotted alexander bennett walking down a baltimore highway. >> when a wanted check is run by a patrolman or an officer that check remains in the computer forever. >> how important was that? >> incredibly important. >> reporter: detectives meyer and childs hopped a flight to denver. it was time to meet this alexander bennett. >> coming up -- >> a suspect's story, a surprise to even these experienced detectives. >> that's why -- it is strange. these birds once affected by oil are heading back home. thanks to dawn, rescue workers only trust dawn, because it's tough on grease yet gentle. i am home, i am home, i am home so find a venus smooth that contours to curves, the smoother the skin, the more comfortable you are in it. flexes for comfort, and has a disposable made for you. skin smoothing venus razors. >> reporter: in january 2012, two baltimore county detectives flew to denver, colorado. their mission -- to track down alexander bennett, the man whose dna had been tied to heidi bernadzikowski's murder 11 years after the fact. >> dna is a good piece of evidence,er ebut we want to fi out if alexander bennett is really a part of this. >> reporter: a day after their plane was wheels down, detective childs was face-to-face with their target. >> alexander, right? >> uh-huh. >> i'm gary childs. how you doing? >> reporter: at first, the detective kept it vague, trying to confirm that bennett had indeed been in baltimore at the time of the murder. the year 2000. bennett said he was. he'd spent about a month on the streets there after being ditched by some friends on their way to a concert. >> i gotta tell you that's strange. >> that's why -- it is strange. >> real strange. and then not to remember anybody you stayed with or hooked up with. >> reporter: then, the detective played his hand. laid out the reason for his visit. >> this girl's fingernails were taken at the time of her death. and under her fingernails is your dna. now there's no denying it. >> reporter: but bennett did have an explanation. and it had nothing to do with committing murder in a house. >> he remembers a confrontation that he had in a bus stop with a female in maryland, in baltimore, right around the time of the murder. >> i got kinda scared because, um, you know i was trying to fight back and i think i hurt her. i'm not sure. >> reporter: the detective didn't buy it. and thought he'd use bennett's story to his advantage. he presented bennett with several photographs, a technique police typically use to help identify criminals. except this time, he was asking a potential killer to identify his victim. could bennett pick out the girl from his supposed fight? heidi's picture was included. >> he knows that he can't give this explanation about having this fight in the bus stop and pick some other girl. so our belief is that if we show him these pictures, that he will pick her. and he does. >> it also, too, kind of looks like her. >> reporter: there was one more crucial detail. remember the neighborhood block watch guy who frightened heidi? the one with the distinctive tattoo? >> do you have any tattoos on your left arm? >> yeah. >> can i see it? >> when i saw the tattoo on his arm and he picked heidi's picture out, i knew it was him. >> reporter: he was the block watch guy? >> he was the block watch guy. >> reporter: but they didn't have enough evidence to book him. so they decided to call in bennett's buddy from that crazy bomb plot, grant lewis, to see what he knew. and lewis was nervous. >> sorry if i'm shaking. i was kind of shaken up. >> reporter: he was evasive about his friend's time in baltimore, but the detective didn't buy his story. then, because lewis had an outstanding warrant, the colorado authorities arrested him. the next day, detective childs kept pressing. >> i think you don't want to tell me certain things because you don't want to hurt a friend of yours. but what i'm trying to explain to you is nothing you say hurts him because what's done is done. >> reporter: at last, grant lewis cracked. he divulged a drunken conversation the two had down by a river after bennett got back. >> he said, i hurt someone bad. and i looked over at him and i said, i don't want to know. and he said, i think someone's dead. i think that's how he said it. i think someone's dead. and then he said, i knifed someone. >> reporter: that was it. corroboration. 12 years after heidi's murder, alexander bennett was charged. he was extradited to maryland to stand trial. heidi's brother frank got the news from their dad. >> it was another one of those things that just brings you to tears just because all that comes flooding back in. it's such a great feeling to feel like finally something's happened. >> reporter: stephen cooke's family was relieved as well. there'd been such a cloud of suspicion around him for so long that news that someone else had been arrested for his girlfriend's murder felt like vindication. >> i was just gosh, i was ecstatic. i was, wow, this is great. >> reporter: this is relief. >> yes. >> reporter: this is what we've been -- >> finally. >> reporter: -- saying for years. it's not stephen cooke. >> i was so excited for my brother. i was just so happy. he can finally put this behind him. >> reporter: two years later, in march 2014, both stephen and heidi's families converged on the baltimore county courthouse for the start of alexander bennett's trial. grant lewis was headed there, too. he'd been flown in to testify. he's going to be your star witness. >> absolutely. >> reporter: but for detectives, the idea that bennett killed heidi all on his own had never made sense. they held out hope that bennett would come clean, but he maintained his innocence. then came the morning of jury selection. >> gary and i are out, getting breakfast. and gary's phone goes off. and he looks down at it. he looks up at me. and he's like, alex wants to talk. and i'm, like, wow. here we go. >> reporter: a heart-to-heart with his mother had convinced bennett to spill everything. prosecutor garret glennon. >> she basically told alexander if he did this, it was time to come clean. that jesus would forgive him. >> reporter: what they call a come to jesus moment was what -- >> it appeared that way. >> reporter: you were beneficiaries of, huh? >> it appeared that way. >> reporter: bennett confessed that he killed heidi. but he hadn't acted alone, he said. he had an accomplice. and that person was who else, but the state's star witness. >> he and grant lewis, as in the bomb scheme, had developed an idea of being contract murderers. >> reporter: grant lewis is the brains of this operation? >> yeah, it was a lack of brains. >> reporter: grant lewis had been sitting in a hotel room preparing to testify. now detectives brought him in and turned the tables on him. >> grant, you're in the middle of this thing. >> i am not in the middle of this thing. >> reporter: at first, lewis denied involvement, but as the detective revealed details from alexander's confession, he started to open up. >> did you send him to baltimore? >> i didn't send him to baltimore, but i know more about this than i've said. >> reporter: lewis ultimately admitted to being involved in a murder for hire scam but said bennett was never supposed to kill anyone. only to get the upfront money before turning the person who hired them over to the fbi. did you believe that story? >> not in the least. >> reporter: so now do you read grant lewis his rights? >> yes. and the cuffs went on. >> reporter: but there was still one major detail left. who hired them to kill heidi? coming up -- >> reporter: what really happened the day heidi died? a first-person account from the killers. >> i was making sure that she was alive. i didn't know. that's when i had the knife. >> reporter: when "dateline" continues. my car insurance with geico. huh. i should take a closer look at geico... geico can help with way more than car insurance. boats, homes, motorcycles... even umbrella coverage. this guy's gonna wish he brought his umbrella. fire at will! how'd you know the guy's name is will? yeah? it's an expression, ya know? fire at will? you never heard of that? oh, there goes will! bye, will! that's not his name! take a closer look at geico. great savings. and a whole lot more. one laugh, and hello so i tried always discreet. i didn't think protection this thin could work. but the super absorbent core turns liquid to gel. snap! so it's out of sight... ...and out of mind. always discreet. for bladder leaks. you get to do the dishes.ed... bring 'em on. dawn ultra has 3 times more grease-cleaning power. a drop of dawn and grease is gone. heare you one sneeze away from being voted out of the carpool? try zyrtec® it's starts working hard at hour one and works twice as hard when you take it again the next day. stick with zyrtec® and muddle no more®. prone jar shatterst livthe competition.pe? olay regenerist hydrates skin better than creams costing over $100, $200, and even $400. fact check this ad in good housekeeping. olay. ageless. if ynow's the time to get your ducks in a row. to learn about medicare, and the options you have. you see, medicare doesn't cover everything - only about 80% of your part b medical expenses. the rest is up to you. so if 65 is around the corner, think about an aarp medicare supplement insurance plan, insured by unitedhealthcare insurance company. like all standardized medicare supplement insurance plans, they help cover some of what medicare doesn't pay. and could save you in out-of-pocket medical costs. so don't wait. call to request your free decision guide. and gather the information now to help you choose a plan later. these types of plans let you pick any doctor or hospital that takes medicare patients. and there's a range of plans to choose from, depending on you needs and your budget. so if you're turning 65 soon, call now and get started. because the time to think about tomorrow...is today. go long. >> reporter: for more than a decade, detectives tried to untangle the mystery of 24-year-old heidi bernadzikowski's brutal murder. on the morning of his own murder trial, alexander bennett unexpectedly confessed to an opportunistic plot. he was really a cash-for-hire hit man. >> all right, alex, you all right? >> reporter: bennett agreed to a deal. tell the truth to investigators and avoid the possibility of getting sentenced to life in prison. interviewed by detective childs, bennett laid out the bone-chilling details of heidi's murder. >> grant was discussing about receiving money to kill somebody. >> reporter: according to bennett, he and lewis had been brainstorming ways to raise seed money for a nightclub. so lewis placed a coded message online, advertising discreet housecleaning services. >> by, discreet housecleaning, grant lewis apparently meant and hoped that someone out there on the internet would understand that to mean that they were hit men. >> reporter: bennett says a client did respond to the ad and offered $60,000 to kill heidi. >> i know from the client, from the person, you know, emphasized to grant and grant emphasized to myself that it needs to look like an accident. >> reporter: bennett's role was the muscle to do the actual hit. lewis, the middle man, communicated with the client and organized everything. in late march 2000, bennett arrived in baltimore from denver. he says he waited for the signal to act. in the meantime, he scoped out his victim, breaking into heidi's home by tampering with the locks, then posing as that neighborhood watch volunteer who so scared heidi. >> i do remember her answering the door. >> reporter: did you go to the door to talk to her more than once? >> it was just that one time. >> just the one time? >> reporter: on april 20th, bennett says he heard through lewis that the plan was a go. after getting into heidi's house he hid behind the front door. >> now my plan was to just try to get her, and like, still make it look like an accident like, maybe snap her neck or something to look like she fell down the stairs or something. >> reporter: heidi walked in through the door, and bennett says he pounced. >> when she came in, she saw me, panicked, i panicked and rushed at her at the front and, you know, tried to muffle her scream. i was making sure if she was alive. i didn't know. that's when i had the knife. and to make sure, had cut her throat. >> reporter: then, bennett said, he wiped the place down. and to throw forensics off he ransacked the bedroom and used heidi's lipstick to make that number one on the living room wall. after that, bennett says, he fled. that's it? >> that's it. >> reporter: heidi's dead. >> heidi's dead. >> reporter: detective childs pressed, who was the client? he showed bennett a photo array of six suspects. assistant state attorney matt breault. >> he immediately separates four photographs and says, it's definitely not these four. and he's left with two. and he takes a couple of moments and stares at them. and eventually, he says, yes, this is him. i remember him. >> this one. i recognize him as the boyfriend. >> reporter: bennett picked out stephen cooke. heidi's boyfriend was arrested for first degree murder. >> deep in our hearts, we all had a feeling that he had something to do with it. >> reporter: stephen's trial began in june 2015. he pleaded not guilty. glennon and breault had the task of convincing a jury that stephen was capable of orchestrating a cold murder for hire. their case would rely heavily on the word of one man, the killer himself, alexander bennett. you really had to believe this guy bennett? >> absolutely. >> reporter: your case was going to rise and fall on that? >> he was going to be the star witness against stephen cooke. >> reporter: bennett told jurors the same story he told investigators, how he killed heidi. video cameras weren't permitted, but bennett's testimony was audiotaped. he decided to confess, he says, because of his faith and for heidi's family. >> i wanted to be a human being. i wanted to give a family some type of peace. i wanted to have faith enough and to grow up into a man and to accept and take responsibility for what i did. >> reporter: but the question still remained. if stephen cooke had planned heidi's murder, why had he done it? according to stephen, they were in love and planning to marry. but some of heidi's friends testified that she was so miserable in the relationship, she was preparing to leave. >> she had asked a friend of hers for a small loan for an easy storage facility in an effort to move out. >> reporter: and the real reason for the murder, the prosecutor told the jury, was pure and simple greed. $700,000. two months before heidi's death, stephen and heidi had taken out hefty life insurance policies on each other. i wonder why an hourly employee at a hardware chain is buying -- >> you should. >> reporter: -- almost a million dollars worth of coverage? >> as we explained to the jury, they had one car between them, which was hers. a red civic. they are not married. they didn't have any children. and they really didn't have many belongings. there really wasn't anything to insure. >> reporter: so once they buy the policy, it's tick tock. >> correct. >> reporter: heidi had no idea time was running out for her. no idea what was going to happen when her boyfriend dropped her home that night. but according to investigators, stephen knew there was a hired killer in the house. >> he set the wheels in motion. >> reporter: driving her home to her death. >> he knew what was in that house. he knew what he was doing. doesn't get any worse than that. >> reporter: but the defense would have an answer for those insurance policies and everything else. and the person to explain it all to the jury would be stephen cooke himself. coming up -- >> the defense, just how strong was the prosecution's case? >> all you have are a confessed murderer's words. that's all. - "once upon a time a little girl learned how to read." her mighty parents kept the adventure going by reading her books that were beyond her skill level. her mind and her imagination grew happily ever after. >> reporter: alexander bennett, the admitted killer on the stand, had spilled out a gruesome account of the plot to murder heidi. stephen cooke, he testified, had hired him via the internet to kill his girlfriend. the motive? the boyfriend wanted her life insurance payout. 700,000 bucks. but stephen's defense attorneys conceded nothing. they thought the prosecution had a weak case overall and one big problem in particular. its star witness, alexander bennett. >> bennett obviously had something to gain by changing his story. >> our old friend, quid pro quo. >> plea deal. >> reporter: tara lacompte and breon johnson were stephen's defense attorneys. they said before the colorado man's so-called confession, bennett faced life in prison without the chance for parole. but once he cut a deal with the state to testify, he could look forward one day to walking free. >> so he's singing for his supper here? >> i couldn't have said it better myself, exactly. the less time he would spend in jail for his own deeds. >> reporter: the defense argued another glaring hole in the prosecution's case was the lack of any physical evidence tying stephen to the crime. the computer in the house that stephen and heidi shared was never taken in as evidence. it's a digital crime at heart. >> not in this case. >> not in this case. >> what happened? >> heidi and steve did have a fairly new desktop computer, and that was never seized by the police. >> so to this day we have no idea what that hard drive on the home computer would've shown? >> no. >> so your guy is being charged with a contract killing, but nobody can produce the contract? >> no. >> reporter: not only did investigators not have any computer records. they didn't have a record of bennett's plane ticket, phone records, credit card receipts. eyewitnesses who saw bennett and cooke together. not even the murder weapon. stephen's sister kim. >> all you have is a confessed murderer's words. that's all. there's not one piece of evidence in this whole entire trial that points to stephen, other than the confessed murderer and these detectives with, oh, their belief. >> reporter: and when it came to motive, they turned the case on its head. the defense version -- this isn't about stephen searching for a contract killer online, but rather heidi look for love on a dating website and finding alexander bennett, who surprised her by showing up in baltimore. >> we thought that he met her online, came here to be with her and she rejected him and he killed her. it made sense to us. it made sense with the forensics. it made sense with the physical evidence. >> reporter: to back up its theory, the defense pointed to this police photo of heidi's keys and bag. if, as bennett testified, he'd jumped her when she walked in the door, how did her door keys and makeup bag end up tidily on the kitchen table in another room? >> so the bag on the kitchen's table's not a trivial thing for you? >> no. i think she made it to the kitchen. i don't think he attacked her at the front door. >> make sure she was dead. >> reporter: and if prosecutors thought bennett was the foundation of their case, the defense felt it had its own star witness, stephen cooke. he would take the stand. >> the bottom line is by this time, we had come to the conclusion that it's bennett versus cooke. it's who you believe. >> for a second, i just looked at her like what in the world is going on. >> reporter: stephen told of finding heidi, the love of his life slumped on their living room floor that night. >> and then i just held her in my arms and was rocking her and crying, just calling her name and all. >> reporter: stephen testified the couple were planning on a future together. and he was surprised when others said that they were finished. >> mr. cooke, you keep saying that you and heidi were going to get married. but we've heard from other individuals that she was thinking about leaving you. >> i've never heard anything like that until now. >> reporter: they were also planning for kids. and that was the reason he wanted a big insurance policy on himself. >> we were going to start a family. we were going to have children. and i wanted to make sure that there was enough money for heidi in case i died. >> reporter: he added it was heidi who wanted insurance for herself. he never pushed her to get it. >> heidi then asked me if it was all right if she could get $700,000 worth of life insurance. and i said, fine. >> reporter: the defense added there was proof heidi was actively nudging along the life insurance policy application. she'd faxed over some final documents just days before her death. it's a fact of the case, even heidi's own brothers harold and frank, can't quite account for. >> there's persuasive stories that she was going to leave him. and yet, there's also pretty compelling evidence that she was trying to get this insurance policy up and running. i mean, she's active in getting -- >> that's the -- >> -- the insurance going. >> yeah. >> yeah, i -- >> that's the biggest mystery to date. >> that's one of the things we don't fully understand. >> reporter: as for the murder, stephen flatly denied ever meeting alexander bennett and said he had nothing to do with heidi's death. >> now steve, you've heard through testimony that you arranged for heidi's murder via the internet. did you do that? >> not at all. not at all. i didn't -- i didn't have anything at all to do with heidi's murder. >> why are you testifying? >> i'm testifying because i want my family and friends and i want heidi's family and friends to -- to know the truth. and for 15 years, they haven't heard the truth. for 15 years, i've been blamed for something i didn't do. i didn't kill heidi. >> reporter: both sides rested. which argument would jurors believe? which man would they choose? the state and its key witness alexander bennett, the acknowledged murderer? or stephen cooke, the man with a good job, middle-class lifestyle and father of a 7-year-old son? the jury was out, but stephen cooke's sister knew the verdict she wanted to hear. >> he's not guilty. they're -- they're going to say, not guilty. they're going to say, not guilty. >> reporter: but after a day and a half of deliberations, that wasn't the jury verdict. >> guilty or not guilty? we find him guilty. >> reporter: stephen cooke was found guilty of first-degree murder, hiring a long-distance killer to murder his girlfriend. >> i felt like i lost my breath. i just was in shock. i just could not believe that's what they've said. i remember screaming, oh, no. >> thank you, lord. prayers are answered. it's relief, joy, and happiness and also sadness because we still don't have heidi. >> reporter: grant lewis, the denver middle man, has already been sentenced to life in prison. both alexander bennett and stephen cooke was sentenced to life in prison without the option of parole. and remember the butcher terry gilliam? police have totally cleared him of any involvement. and in fact offered him an apology. >> it's not everything, but it does -- it's a start. it does mean something. >> reporter: as for heidi's family, their days are all about the kids and grandchildren. and parents who believe devoutly in a hereafter, always remember, of course, heidi. >> did you talk to heidi? >> oh, yes. i did. >> what did you tell her? >> i just said, sweetie, we finally got the answers we've been praying for. and justice will be served. >> that's all for this edition "dateline." i'm lester holt. thanks for joining us. probe. chilling video of a woman found chained by her neck after being held captive by a serial killer. one of the rescuers is speaking out. >> plus historic wins on the ice and the clay. "early today" starts right now. good morning. i'm gigi stone woods. >> i'm frances rivera. breaking overnight attorney general of maryland in washington, d.c.,