>> i'm glad everything's okay. we'll disregard for you. have a good night. >> reporter: it was probably the last time peggy's voice was heard by anyone except her killer. police were convinced -- still are convinced -- peggy was letting in someone that she knew. susanne? maybe. but why would she use a tracfone to get her mom to open the door in the middle of the night? >> she wouldn't have to let me in. i have a key. and i know the -- how to work the alarm. >> reporter: right. so if you wanted to kill her, you could have waited until she went to sleep -- >> and went in. >> reporter: unlocked the door -- >> and went in. >> reporter: --disarm the alarm. >> there you go. there you go. >> reporter: police had warrants for all the family members' phones. and a few weeks later they had boxes of records to comb through. detectives analyzed susanne's first. what could you tell about susanne from her cell phone records? >> a lot. >> reporter: investigator robyn arias from the westchester county d.a.'s office specializes in phone data analysis. >> i figured out a pattern of susanne's life just based on her records.