reporter: police even looked into susanne s shopping habits and found something that intrigued them. susanne bought her groceries at a nearby shoprite, a big supermarket chain. in fact, she was a creature of habit, visiting there 22 times in the month prior to peggy s murder. but on the 23rd time, the night before peggy s murder, susanne shopped at a different store, the one near her mother s house. what did she buy? she bought cleaning supplies, rubber gloves and some other items like that. reporter: had the crime scene been cleaned up? detectives didn t think so. but still found it suspicious. just before your mother was murdered, you go to the shoprite. not the one near your house. the one near your mother s house. mm-hmm. reporter: and you buy rubber gloves and cleaning supplies. i did. i mean, i buy cleaning supplies all the time. i have a cleaning girl. she probably needed rubber gloves. she probably needed floor cleaner. that s interesting.
corporate executive, caring mom, a million dollar smile and a $4 million estate. oh my god! i need an ambulance! murdered. i went screaming out of the house. this was a prominent woman murdered in her home late at night. almost certainly someone she knew? correct. where would it lead? a twisted case of betrayal and greed. the finger is being pointed at me. her own daughter, a suspect from the start. you pull the knife out and you tell the police, oh, you re going to find my fingerprints on the knife. her son-in-law, who seemed so devoted. they interrogated us for eight hours. or maybe someone else with a million dollar motive? what s going on here? how many people are involved in this? devious. it s an evil mind. someone just doesn t turn into a murderer.
reporter: detectives obtained data dumps from cell phone towers in washington, d.c., that allowed them to map the travel of the cell phones as they pinged off the towers. can you follow diana s cell phone? it bounced all around washington, d.c., simultaneously with andrea s. they were together the entire day. reporter: by late friday, the night peggy was murdered, diana s phone stopped moving and went quiet. but andrea s phone? we saw andrea s phone head out of the washington area and head up i-95. reporter: exactly the way you would drive if you were heading for peggy s house? right. reporter: along the way, the tracfone was activated. and at 1:17 a.m., it pinged a cell tower right near peggy s house. what was going on? was this murder for hire? did andrea benson drive up to new york, call peggy on the tracfone, somehow get into her house and murder her?
reporter: susanne, who lives in a suburb of new york city, called diana, her sister-in-law in florida. and i said, have you spoken with mommy this morning? she said, you know, i called her at 7:30 and i haven t heard back from her. reporter: now worried, susanne got in her car and drove to peggy s house. as i m driving, i m calling the house, the cell, the house, the cell. no answer. and now i m getting panicky. i pull into her driveway, and i saw the roof of her car through the garage door. and i said, oh, this is not going to be good. reporter: she was right. susanne says she found peggy crumpled at the bottom of the stairs with a knife in her chest. and the first thought was, oh, my god. she tripped on the cat and fell and stuck herself. reporter: then she says she saw the blood. and i said, oh, my god, she must have hit her head when she fell. reporter: and then susanne did something that would later raise a number of questions. i pulled the knife.
says, okay, i m in? for $10,000. reporter: and right there the deal s done in that car? and the plan starts taking shape. reporter: benson said she drove the four hours up to peggy s house, and then diana used that tracfone and called her mother-in-law. diana tells her, i m here. don t you miss me? i want to come see you. reporter: at 1:30 in the morning and peggy lets her in. benson said she went in with diana. they were wearing hair nets and gloves to keep from leaving evidence. diana and peggy sat across from one another and talked. benson said at one point, peggy went upstairs, perhaps to call for help. she and diana followed her. then, as peggy came back down stairs that s when andrea benson started choking her. reporter: andrea said peggy was still alive when diana hit her mother-in-law with a metal ball and beat her with that statue head.