. >> reporter: investigator joe maura. >> that was like leaving the gun at the scene with your finger prints on the gun. it's the same exact thing. >> reporter: detectives began tracing that virtual i.p. address. and it led them to a suburb just outside boston. >> that address led police to an apartment in quincy. 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 li

. >> reporter: joe maura, a boston-based private investigator and "48 hours" consultant, says that byng as her own boss, trisha was increasing her risk. >> there's a street prostitute. she's going to have a pimp down the street or across the street or on the corner protecting her. somebody using craigslist, you know, getting a fancy hotel in boston, she's on her own. >> reporter: but that april night, when trisha saw the man who answered her ad, she felt perfectly safe. >> he looked nice. he looked young. good looking, obviously. >> reporter: but once the door was closed, this good-looking young man pulled out a very dangerous-looking gun. >> i backed up a little bit. he just said, "you know, if you do everything here asked, no harm's going to come to you." and he said, "lay down. put your hands behind your back." and then when he started to walk towards me he put the gun back in his pocket. >> reporter: the man put on some black leather gloves and took out some plastic zip ties like these. >> he kneeled down with one knee in between my legs and took my wrists and put the tie around o