dad... see that? oh, yeah. good, son. so, no, i need some answers... brendan mccray. i got to go. fbi. we need to talk to you. about what? your exploits on the internet. cheerchik15-- ring a bell? turn it around, put your hands behind your back. okay, wait, wait, this is nuts. guys, we're on the same side. the same side? i can explain it. you just need to... sir, you don't want to do this here. brendan: hey, call your mother. i need the lawyer. do you have an irregular heartbeat called atrial fibrillation, or afib, that's not caused by a heart valve problem? are you taking warfarin to reduce your risk of stroke caused by a clot? you should know about pradaxa. an important study showed that pradaxa 150mg reduced stroke risk 35% more than warfarin. and with pradaxa, there's no need for those regular blood tests. pradaxa is progress. pradaxa can cause serious, sometimes fatal, bleeding. don't take pradaxa if you have abnormal bleeding, and seek immediate medical care for unexpected signs of bleeding like unusual bruising. pradaxa may increase your bleeding risk if you're 75 or older, have kidney problems or a bleeding condition, like stomach ulcers. or if you take aspirin products, nsaids, or blood thinners. tell your doctor about all medicines you take, any planned medical or dental procedures, and don't stop taking pradaxa without your doctors approval, as stopping may increase your stroke risk. other side effects include indigestion,stomach pain, upset, or burning. if you have afib not caused by a heart valve problem, ask your doctor if pradaxa can reduce your risk of a stroke. for more information or help paying for pradaxa, visit pradaxa.com. ♪ i'll stop the world and melt with you... ♪ pure joy. pure togetherness. pure delicious chocolate. pure hershey's. seriously smooth and in excellent condition... those are your lips with covergirl lip perfection. it doesn't just give you beautiful color now, it gives you more beautiful lips in 7 days. in a single stroke, rich color rocks your lips. in a single week, the silk moisturizing complex helps smooth and soften them. how perfect! 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"can i come over?" cheerchik15: "parents are away for weekend. i'll get pretty for you." santos: "i'll be there." yeah, yeah, that's my user name, but it's not me. right. you going to tell me it wasn't you three months ago? huh? god, you don't know what this is about, do you? no, i get it. i understand-- it's payback for what happened to your daughter, isn't that it? my daughter? i mean, it makes sense. i think there is a serious mistake... because you couldn't protect her... you're the ones who couldn't protect her! what? the guy who lured her out was on parole. i belong to a group called parents stop predators. we go online to chat rooms, we pretend to be teens and we have these creeps arrested. say what? maybe someone hijacked my user name from the group, i don't know, but whoever killed these men it wasn't me. brendan? are you all right? no, it's fine. everything's okay. i just saw the lawyer. he said that the fbi was talking to you about some murders? it turns out it was all a big mistake. a mistake? yeah. come one. you people are always making mistakes. i'm sorry, ma'am. you know, if you were doing your job my daughter would still be alive. let's go. not sure this is going to brighten the mood any. his story checks out. local p.d. confirmed that three months ago he attempted to lure victims to a house. they didn't bite, so he sent a letter instead. saying? notification that they'd attempted explicit sexual conversations with a minor. threatened to out them to their families if they didn't seek counseling. oh, and he has an alibi to the last murder. charlie: don. what, you got something? yeah, i think we figured out why the killer's picking these houses. amita's got the laptop. so we hit a wall trying to composite our killer's ideal house because we realized he's not choosing houses. he's choosing by neighborhood. exactly. we're all familiar with megan's law. yeah, requires the police to tell the public about sex offenders. there's also something called jessica's law. right. where they can't live. not 2,000 feet from a school, or day care, right? so what we did was we ran an analysis of jessica's laws hot zones and mapped it against the locations of the three houses. the houses where the murders occurred are all dead center? right. so these are the areas we're supposed to look? and i also mapped all the other jessica's law hot zones in the entire l.a. area. oh, boy. come on, charlie, what are you kidding me? there's no way we can cover this much ground. megan: wait a minute. that might be the killer's message. that there's no way we can stop these guys. only he can? a web host monitoring one of their sites just gave us a call. our killer posted a video. don, you have to see this. i didn't do anything. i just came by to see a friend. (panting) my name is john santos... i came to this house to have sex with a 15-year-old girl. this isn't the first time. david: he beats them until they confess. i've had relations with over 30 girls under the age of 15. i ask for forgiveness from them... from their families... you don't want to see the rest. killer uploaded this video to a site that holds suicides, beheadings, you name it. saw this today? yeah, this morning. i did a search and found a similar video of victim #2 put up two days after he was killed. what about the first victim? nothing yet. all right, well, can you trace him? techs have been trying, but the killer must have used some kind of masking program. they haven't been able to crack it. all right, well, who are you going to call? charlie. yeah, so it seems to me that the killer is using some kind of "onion-routing" technique. onion-routing is a way of sending content over a network and still remaining completely anonymous. it's sort of like a person sending a series of letters inside letters. he sends the first one to a friend, who then finds a second letter inside, and all that friend knows is, he's supposed to forward that letter, so he does. so, each letter is like a layer of an onion being peeled back until the final message inside is revealed. you think you can crack something like this? you know, it's funny, i created an algorithm for one of larry's projects, and so i-i think i can adapt it to this case. and speaking of larry, charlie, i-i... i keep thinking that maybe there's something that-that we can do to help him. i keep thinking the same thing, but i-i don't know what. what about all the people you know in the science community? what if you call one of them? (sighs) well, larry's noisetamper project is an nsa initiative, but you have to understand, if nasa's questioning larry's overall fitness... well, then we should make that call, because larry is devastated. his spirit is-is crushed. he's like a star collapsing in on himself. you two have been hanging out a lot. charlie, i don't want him to go either. but if there's anything we can do, we have to let larry be larry. the killer's timeline is definitely accelerating. he's gone from six weeks between kills to ten days; now down to 72 hours. what's the deal, is charlie's thing up and running? megan says it'll be up in an hour, but there's a problem. what's that? it can't run a trace after an upload's completed. it can only look for content during transmission. great, so what do we do, wait till the guy does it again? this better work. (typing) charlie: hey, so i got the algorithm set to scan the site the killer's been uploading to. once we get a hit, we'll be able to trace his transmission. all right, good. (beeping) we got a hit, professor. hey. don, hey! we got a hit. my name is wendell henderson. i came to this house to have sex with a 15-year-old girl. in the past ten years, i've... i've molested over 40 young girls. all right, so? it's getting there. just another minute. henderson: i understand that i can never give these young girls... wow, this is really odd. what? this transmission-- the file size and video times are steadily increasing. the checksums are constantly changing. this is uploading live. wait, what? don... ...their trust... he hasn't killed him yet. tina, come on. you gotta give me a location now. tapping into the server. i should be able to run a ping trace to find the broadcasting socket. call me in the car. got it. 252 estrella. ...the pain and suffering that i've caused... (sirens wailing) jimmy, eric, you gotta come with me. colby, david, take two and head out back. fbi! fbi! fbi! fbi! fbi! wait here! don't move! what's happening? you the owner of the house? yes, i don't understand. all right, all right, it's okay. you've been on the internet? no, it's been down today. okay, it's okay. just relax. we're sorry, it's the wrong house, ma'am. we're going to rectify this. the killer's covering his tracks. he must have piggybacked their router. i'm sorry, sweetie. hey, charlie, we got a problem here. (muffled scream) yeah, don, what's going on? we... we just lost our video transmission. don: it's the wrong house. he must have piggybacked the signal. (mutters) you know what, if he did, then he's in the area using a wifi signal. tell me all the houses for sale in the neighborhood. i'm already on it. okay, i've got a house. it's two streets over. it's 2463 north irving boulevard. oh, damn. damn! ah, he's dead. he must have heard the chopper. left in a hurry. ah, he left this-- it's computer transmitting gear. we were close. yeah, well. you guys clear? agent: all clear. ice breakers frost. a great tasting mint core, frosted in powerful cooling crystals. ice breakers frost. feel the frost. real restaurant style flavors like loaded potato skins. hey, funky party people. are we having fun? [ male announcer ] any more "restaurant" and they'd come with an annoying waitress. new snackers from ♪ hot pockets new snackers from so to save some money, i trained mathis team of guinea pigs to brrow this tiny boat. guinea pig: row...row. they generate electricity, which lets me surf the web all day. guinea pig: row...row. took me 6 months to train each one, 8 months to get the guinea pig: row...row. little chubby one to yell row! guinea pig: row...row. that's kind of strange. guinea pig: row...row. such a simple word... row. anncr: there's an easier way to save. get online. go to geico.com. get a quote. 15 minutes could save you 15% or more on car insurance. [ screaming ] [ zapping ] there goes dwayne's car. oh, man. there goes dwayne's house. whoa! whoa! and there goes dwayne. man, that thing does not like dwayne. [ male announcer ] state farm's got you covered. nice landing. it was. [ male announcer ] get to a better state. nice landing. wendell henderson. was. 32, computer geek. same story. he talked dirty with brendan mccrary in a chatroom three months ago. and the killer contacts him again, using mccrary's username to lure him. electronic gear we found was all bought with cash. the store owners can't remember who made the purchases. i mean, so whoever it is clearly has access to mccrary's files. colby: he may be right. it may be somebody from his group who hijacked his username. i have a list of all the parents stop predators volunteers, transcripts of their chatroom conversations. well, then may i take a look at that? a lot of names to go through, charlie. maybe instead of going through the names, why don't we go through the chat? what do you mean? well, it's just like any language. you know, on-line messaging has its own vocabulary and syntax. senders of these messages develop their own unique patterns of speech, which can then be analyzed using statistical linguistic analysis. it's like a jeweler beading a necklace. a jeweler chooses certain types of beads then strings them in a particular pattern according to his own unique, personal style, just like im senders will exhibit their own styles, patterns of speech which are unique to them. so, then we just compare them. yeah, to the transcripts of the volunteers for this "parents stop predators" group. look, if the killer is in that group, he will be betrayed by his own chat. amita. hey. hi. um, have you seen charlie? i-i have the transcripts from the killer's latest chat. he's working in larry's office. larry's at a symposium. and he thought he could concentrate better there. oh, he's got you working on this, too? well, my experience with asymptotic combinatorics comes in handy. sounds like everything you need for a good working relationship. charlie and i work well together. well, that must be nice. you gonna miss him, aren't you? yeah, a little more than i think i originally thought i would. well, he's coming back. yeah, he is. and i have a telescope now. (laughs) you know, i'm actually going that way. i'll walk with you. (laughing): okay, thank you. sure. charlie: i need to speak with dr. york concerning lawrence fleinhardt. f-l-e-i... oh, you know him? okay, well, then i guess you're the person i need to be speaking to. um, i've known professor fleinhardt for approximately 17 years, and i'll be the first to admit he's an eccentric. i'm aware that that's your concern. yeah, but he's brilliant. he's the kindest, most decent person i know. yes. i am, i am personally vouching for him, yes. well, i appreciate that. uh, yeah, yeah. yeah, i guess that is all i have to say. thank you. (mouthing) hey. so, got something? yeah, i did what i said i was going to do. i compared the killer's messaging patterns against our parents stop predators group and i've isolated a near match within an error margin of five percent. all right, so who? according to my math, brendan mccrary. no, no, that's not possible. he's already alibied for two of the murders. yeah, but charlie's saying he found a near match. you know they say people and their pets start to look alike? same thing happens in family dynamics. one member of a family will start to think and speak alike. what, the family? the wife did go off on you pretty good. yeah, but going off isn't what i'm talking about, 'cause that's too explosive. this kind of rage is, is calculated carefully. all right, well, if you don't think it's the wife, there's only one survivor left. all right, matt, this is from your laptop. list of sexual predators, these are chat room conversations. he says you downloaded it from my computer. is that true? four of the names from your father's list were murdered. now, he didn't do it. you're the only other source. you going to tell me what i want to know? agent epps asked you a question, son. i didn't kill anyone. who did? hey, look, it makes sense that you'd want to get back at people like that-- what happened with your sister. it's normal, it's natural. i would, too. but that makes you just like them, right? is that what you really want to be? no. sweetie... i didn't know they were going to get hurt, i swear. tell them what they need to know. i met this person in a chat room for families of victims of sexual predators. i didn't know his name or anything about him. all i know is that he listened. honey, if you were having problems, why didn't you come to us? what were you going to do? matt! oh, seriously, dad, you're too busy with your catch the creep group. and you? you don't even come out of your room half the day except to go into katie's bedroom to look around. you don't think i see? i loved katie. losing her sucked. but i didn't just lose her. i lost you, too. this person you said you met, what'd you talk about? my dad's group, mostly. how useless they are getting these guys into counseling. how counseling doesn't do anything. you gave him the names, right? it's... i didn't mean for any of this to happen. (sniffles) i'm sorry. larry: hey, charles. i thought you should know. yeah? i heard from nasa. uh-huh? i'm back on the mission. oh, that's... that is great. congratulations. well, apparently, someone at the nsa did some arm twisting. oh, well, they're the experts, off the record. well, on the record... thank you very much. i didn't do anything... aside from making them see the error of their ways. consider it my contribution to national security. (laughs): what, you think having me off planet improves our security? i didn't say it. well, i know you don't want me to go. and therein lies the nobility of what you did. what i did was repay a debt. you don't remember this, do you? but, uh, when i was a 14-year-old kid at princeton... when we first met, do you remember what you said to me? as i recall, i said many things. you said... that you wanted to see me safely launched into the academic firmament. now it's your turn. listen, just so you know, it's always harder on the mentor than it is on the student. colby: i traced the username of the person that matt mccrary talked to in that chatroom. it was a dead end. the guy keeps covering his tracks, right? how about you, you got anything? i keep going over my interview notes. matt mccrary had a conversation with the killer talking about how useless counseling is for guys like these. sounds an awful lot like my conversation with elaine tillman. the first victim's wife? you think she had something to do with this? she told me her husband swore he would never touch their kids. what, she lying? we never did find his video confession online, and i think i know why. because she didn't need a confession. right, 'cause she already knew. all right, so going with that theory, i get why she kills her husband, but what about the rest of the guys? the notification letter brendan mccrary sent to the tillmans must've been the trigger that just pushed her over the edge, and then she met matt online. and she knew that there were all these other men out there just like her husband. we know scott tillman taught computer science in high school, so she could've learned what she needed to know from him. architect of his own demise. over the past three months, she's contacted a half dozen real estate agencies. she did say her and her husband were looking to buy a bigger house. she used a different name to contact the real estate agent, make the appointment. all right, where is she? two hours ago, she dropped the kids off with a neighbor, said her mom was sick, had to take her to the hospital. uh, no, she's lined up her next victim. all right, how many houses are in here? almost 100. yeah, at least. all right, let's get it up to charlie. so i mapped all the locations of the houses on elaine tillman's list. and assuming she's sticking to live uploads, she's gonna need a strong wifi connection. all right, i don't want to go to the wrong house again, so... doesn't even matter because this time we'll know where she's transmitting from. yeah, and how's that? when i did my real estate analysis, i was looking at wifi connectivity. in fact, i had your techs cross check the database of broadband users in these areas against owners of wireless routers. that sounds like a lot of houses. not where we're looking. because, remember, she was choosing houses in jessica's law hot zones. overlaying those hot zones over our map of houses and factoring in for wifi connectivity, i've isolated five locations where she'll likely strike next. all right, i'm gonna get lapd into those areas. as soon as we find her car, we're gonna move in. there's elaine tillman's car. house is around the corner. 2152 floresta. and lapd has the perimeter? let's take the house. (man yells) what do you want from me?! everything! fbi! fbi! fbi! fbi! one more step and he's dead! elaine, nobody's doing anything. nobody's gonna move until you and i talk. about what? you know why i'm here. you know what i have to do. i know what happened. i know your husband hurt your daughter. he lied to me. he promised he'd never touch susan! okay, i want you to think about your kids. your children do not want you to do this. this man never touched your daughter. what about the other daughters?! i have to stop them! no! what difference does it make? if i kill him or not, they're gonna shoot me anyway. elaine, no one's gonna shoot you. listen to me, we have proof. we have his words on his computer. we know why he came here. he's not walking out of here a free man. put down that knife and he's going to jail. and there's nothing you can do to him that isn't going to be worse in there. (crying): just don't... just... okay. you okay? yeah.