remember trisha leffler's stolen underwear? >> the boston detective called me from his apartment.he said, "we did find both pairs of your underwear." >> reporter: and police report finding two more pairs of women's underwear hidden under markoff's mattress, though they haven't said whose. but if markoff was keeping underwear from his victims as trophies, it was beginning to look like these were more than just simple robberies gone wrong. >> it wasn't just about the money. it was, "if i'm going to get the money, how can i get it and fulfill this personal need for thrill?" >> reporter: jordan thinks markoff gets his thrills from a frightening mixture of power, sex, and control. and one night, back in college, after more than a few drinks, morgan houston may have caught a glimpse of philip markoff's thrill-seeking. >> going up into the tower where i lived, he cornered me and pushed me up against the wall and was trying to kiss me, and i was saying, "no, phil." you know, "we're just friends. what are you trying to do?" i was trying to push him away, and he was being forceful and i cou

my name is trisha leffler.and i was attacked by the craigslist killer. >> craigslist, in the prostitution game today, is the old yellow pages. >> it's easier for aspiring predators to find prey. >> click on the site, and you've got your victim for the night. >> julissa was 25 when she died. she was young and she was beautiful, but, unfortunately, i think she went down a very troubled path. her services are advertised as massages, massages of a more sexual nature. julissa was a fighter. she wouldn't let anybody rob her or take her money. she fought back. her killer, who police have identified as philip markoff, smashed her over the head with the gun, and then shot her three times, striking her in the heart. >> reporter: boston police say the craigslist killer is a 23- year-old premed student at boston university. >> i discovered that philip markoff did have a secret life, a side of him that his family and friends had no idea existed. >> he was participating in these sexual web sites. he posted photos of his nake

. >> reporter: trisha leffler believes she's lucky to be alive after she says phillip markoff answered it and that's when he pulled out the gun. >> reporter: police say markoff was addicted to websites special specializing in sex. >> he was convince that had to one would ever suspect that he had this double life, that he would actually answer ads for sex workers or erot tick dancers. >> reporter: but his propensity towards violence was never evident up the night he tied up trisha and robbed her at gun point. >> it dawned on me later that he could have very well killed me. >> reporter: police say that's what did he to jew lisa brisman fur days later. >> the co-authors of the "seven days of rage." and they join us live in the studio this morning. how is it, that was one of the mysteries, a continuing mystery, this this guy did seem to leave a double life. >> absolutely. he had a lot going on. he had a lot of sexual profiles online that no one knew about. we sort of reveal them now for the first time in the book and this broadcast on saturday night, "48 hours." but nobody knew about him.