specifically to an apartment belonging to philip markoff. >> reporter: philip markoff-- for the first time investigators could match a name to a suspect. and they were shocked to discover this 23-year-old was a second year medical student at boston university. >> philip markoff is a brilliant student. we know he has an uncommon mind. >> when they got philip markoff's name, they still weren't convinced. >> at that point, they put him under surveillance. >> reporter: officers from boston's elite fugitive squad began an around-the-clock stakeout. >> they began their stakeout late saturday night, early sunday morning, waiting for somebody to come out that would look like the man in these video surveillance photos. and when they did they said, "okay, we have somebody who looks like these photos." one sergeant basically called the investigators after watching him all days and said, "i like him. i like him a lot." >> reporter: but before the cops brought him in for questioning, the d.a. wanted a positive i.d. they reached out to the craigslist killer's first target, trisha leffler. >> i got

." >> philip markoff is a 23-year- old boston university medical student-- tall, blonde, good- looking gentleman, looked nice. he was by all accounts what we would consider a good citizen. i just said, "hi, how are you? and he said, "good." i said, "all right, come on," and motioned him to follow me into the room. >> he has no criminal record. he has a fiancÉe. he seems to be an ambitious, intelligent man who had a future ahead of him. >> i shut the door. i turned around and i locked it. that's when he pulled out the gun. he's very calm, like-- like he's done it before. 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