>> andrea benson. >> reporter: how do you know it's andrea benson? car, registered to her. >> reporter: you can read license plates with those cameras? >> they're pretty good. >> 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? a

lisa: when andrea benson worked as a prostitute in portland, oregon, she knew that the clients wanted young. andrea: even when i was in the life, i put that my age was 19 and guys would just be like, "well, you don't look like you're 19. you look like you're younger. you look like my granddaughter. you look like my daughter." lisa: benson claims she was trafficked on backpage, an online classified advertising site. carol robles-roman: this is a business model that makes $9 million a month using this exploitive model. lisa: carol robles-roman heads lega backpage on behalf of a "jane doe" client who claims to have been trafficked on backpage, and two other advocacy groups who support victims of sex trafficking, including children. carol: these are kids. these are young kids, these are minors and there's no justification, there's no way to defend it. lisa: no one alleges that backpage is running a prostitution or trafficking ring. sen. claire mccaskill: they did not turn away ads selling children. lisa: however, in january, a senate homeland security subcomittee on investigations conclud