(Updated 08.20 PST with quotes from the accused's fiancee)
On Friday, Iwho appeared to be using Craigslist to meet masseuses.
Police said they believe he is responsible for the murder of Julissa Brisman, at the Marriott Copley Place in Boston, as well as several other incidents at Boston hotels.
On Monday, police announced that they have arrested Philip H. Markoff, 22, in connection with the murder. He is a premed student at Boston University, a school spokesman confirmed to the Boston Herald.
Markoff has been charged with murder, unlawful possession of a firearm, and kidnapping.
The Boston Herald quoted Boston Police Commissioner Edward Davis as saying: "We are very, very happy to have this man off the street in such a timely way."
Markoff's fiancee, 25-year-old Megan McAllister, told the Herald on Tuesday: "I will stand by Philip as I know he is innocent."
On an unnamed social networking site, the Herald reported that she wrote: "Philip is a beautiful man inside and out (...) He is intelligent, loyal, and the best fiance a woman could ask for. He would not hurt a fly!"
She has also been e-mailing ABC News. Ms. McAllister is due to marry Markoff on August 14.
It appears that computer experts were among those police turned to in order to track down the suspect.
Craigslist had already stated in these comment pages: "There appears to be a psycho on the loose around Boston. We always help the cops out fast with the help they need from us, but they tell us not to comment on current investigations."