BOSTON -- The medical student charged as the "Craigslist Killer" pleaded not guilty in a Boston courtroom Monday to charges that he attacked women who advertised erotic services on the Web site, shooting and killing one of them.

Philip Markoff, 22, of Quincy, Mass., a second-year Boston University medical student, was charged by Boston police with last week's slaying of JuLissa Brisman, 26, of New York, a masseuse who was found shot to death in an upscale Boston hotel.

Markoff, who was shackled in the courtroom, showed no emotion as he was ordered held without bail on the charge.

Suffolk County District Attorney Dan Conley called Brisman's killing "savage," saying Brisman's killer was a "man who's willling to take advantage of women, to hurt them, to beat them and rob them."

"This poor young woman, JuLissa Brisman, no matter what choices she made in life, she's a human being," Conley said after the arraignment.

A search of Markoff's home revealed guns and plastic zip-ties, the district attorney's office said.

Conley said authorities believe the initial motive in both cases was robbery. Sources said Markoff may have been trying to pay off gambling debts. Markoff and his fiancée had listed the Foxwoods casino as a possible destination on their wedding Web site.

Brisman's family was at his arraignment in Boston Municipal Court Tuesday. They declined comment.

Brisman and two other women who were attacked had all been bound with plastic zip-tie cuffs, police said.

State prosecutor Jennifer Hickman said at Markoff's arraignment that the woman who was robbed Apr. 10 at the Westin Hotel brought a man back to her hotel room sometime after midnight. As soon as they entered the room she turned to find him pointing a gun at her stomach and she was ordered to lie on the ground. Her assailant then bound her wrists with plastic ties and searched her belongings, taking cash and credit cards. He then tied her to a door and put duct tape over her mouth before leaving.

Brisman had advertised her massage services on the site. She was killed at the Copley Marriott hotel while resisting a robbery attempt, police believe.

The woman involved in the Westin Hotel attack said during a phone interview from Las Vegas that she probably survived because she did not resist.

"I just complied with everything he wanted me to do," she said. She asked that her name not be used. She expressed sympathy for Brisman's family.

"It's crazy to ahve to go through that. My parents don't know what I do, so to find out that way would be horrifying," she said.

Markoff remains a suspect in an attack on another woman at a hotel in Warwick, R.I. who also offered services on Craigslist. The suspect was captured on surveillance photos at all three crime scenes, police said. In several photos, he appears unconcerned as he types on his BlackBerry. Investigators said that BlackBerry is what led them to Markoff.

Conley said Markoff obtained a new e-mail account the night before he allegedly met with Brisman. They said that e-mail led them to his Internet Service Provider (IP) address, and from to his apartment address in Quincy. They said at that point police undertook surveillance until they were able to place him under arrest.

Police said that they believe these may not have been the only attacks on women offering services on Craigslist.

"Our priority is to hold him accountable for each and every one of those crimes, not to hold you accountable for offering services on Craigslist," he said.

Last Thursday night, a Las Vegas woman was bound and held at gunpoint at the Holiday Inn Express hotel in Warwick by a man who answered her Craigslist ad. Her assailant fled when the woman's husband returned to the room.

In surveillance photos released Monday, a white man can be seen walking into the lobby of the hotel and walking up the stairs.

A break in the case may have come from the account of that 26-year-old exotic dancer who was offering lap dances for money at the hotel through her Craigslist posting.

She described to police the striking similarity between her attacker and the man wanted for the Boston attacks. She described her attacker as about 6 feet 3 inches tall with blond hair.

On April 10, the 29-year-old prostitute from Las Vegas was robbed at the Westin Hotel in Boston. She was tied with a plastic cord and robbed of a debit card and $800 cash.

Boston police said investigators had followed up on more than 150 tips received since surveillance photos of the Boston hotels were released last week.

"They followed high-tech leads and they used old-fashioned shoe leather. They connected computer IP addresses to physical locations," Conley said.

Markoff was arrested after being pulled over on Interstate 95 in Walpole, Mass., Monday afternoon. Police said he did not resist arrest.

Brisman was a model for Near Ear, and was featured as the "Cell Phone Girl" in a video produced by the New York company, according to the company's Web site. Brisman also did some acting and model roles for Spike TV, the Web site said.

Brisman's friends said they didn't know she had advertised massage services online.

Matthew Terhune, 34, a photographer from New York City, said Brisman told him she had been paid $1,000 to fly out to private parties in Chicago and walk around in a bikini or topless. "But I don't think it went beyond that," Terhune said. "They were just parties where guys wanted to see hot girls."

Boston University said Markoff had been suspended from its medical school.

Markoff's attorney, John Salsberg, did not immediately return a message left at his office on Tuesday morning.