BOSTON, Massachusetts (CNN) -- Friends and acquaintances of Philip Markoff, a medical student accused of killing a woman he may have met through a Craigslist online ad, described the 23-year-old as a model student.
Medical student Philip Markoff, 23, appears in court Tuesday in Boston, Massachusetts.
"My girlfriend actually rode the elevator with him a lot alone; it's kind of freaking her out now," said Patrick Sullivan, who lived in the same apartment building as Markoff in Quincy, Massachusetts, a Boston suburb.
"She thought he was kind of the all-American, good-looking guy," Sullivan said. "When she saw him on TV yesterday, she even remarked, 'I can't believe it's him. I always thought he had such a great smile, and he was so nice to me.'"
James Kehoe, a friend of Markoff's from the State University of New York at Albany, where both attended college, said Markoff was "one of the best students I've probably ever encountered."
"He would never put anything in front of his work," he said. "He had great aspirations to be a doctor."
Markoff, a second-year student at Boston University's School of Medicine, is charged with killing 26-year-old Julissa Brisman of New York on April 14 at Boston's Copley Marriott Hotel. Watch acquaintances describe a smart, good-looking guy »
Markoff was arraigned Tuesday and is being held without bail.
His attorney, John Salsberg, told reporters after the hearing that Markoff is "not guilty of the charges. He has his family's support. I have not received any document or report or piece of evidence other than what I heard in the courtroom. All I have at the moment are words -- no proof of anything."
A Boston University spokeswoman said the school suspended Markoff, who has no criminal record, when it learned of the charges on Monday.
"This was a brutal, vicious crime. Savage," Suffolk County District Attorney Daniel Conley told reporters. "And it shows that Philip Markoff is a man who's willing to take advantage of women -- to hurt them, to beat them, to rob them."
A procedural not-guilty plea was entered on Markoff's behalf, Conley said.
Evidence from the scene suggests that Brisman "put up a fight," prosecutor Jennifer Hickman said during the hearing Tuesday. Police have said that Brisman, a model, advertised as a masseuse on Craigslist, a popular online classifieds service.
She suffered blunt head trauma, Hickman said, and was shot three times at close range. One of the bullets passed through her heart, killing her, she said.
Markoff is also charged in connection with the April 10 robbery of a woman at a Westin Hotel in Boston. In that case, the woman made arrangements to meet a man through Craigslist for a massage at the hotel, but was held at gunpoint and bound, Hickman said. She was robbed of $800 and personal items and left tied to a door handle with duct tape over her mouth, the prosecutor said in court Tuesday.
The victim was not identified.
Megan McAllister, who identified herself as Markoff's fiancee, maintained his innocence in an e-mail sent to ABC News. McAllister said Markoff "is the wrong man" and "was set up."
"Unfortunately, you were given wrong information as was the public," she wrote. "All I have to say to you is Philip is a beautiful person inside and out and could not hurt a fly!"
She accused Boston police of "trying to make big bucks by selling this false story to the TV stations. What else is new??"
Authorities believe the motive in Brisman's death was robbery, Conley told reporters. In executing a search warrant at Markoff's home, police found a firearm, along with restraints and duct tape, he said.
Surveillance videos from the hotel where Brisman was killed showed a tall, clean-cut young blond man in a black windbreaker leaving the property, according to Boston police, who had sought public assistance in identifying the man.
Police traced the Internet communications with Brisman to an e-mail account that had been opened the day before her death, Conley said. Using Internet provider information, they found the computer was at Markoff's residence in Quincy. Authorities put the home under surveillance, Conley said, and "the case just begins to build from there."
Markoff, meanwhile, is "bearing up," Salsberg said. "It's obviously a difficult time for anybody in these circumstances with the charges that have been brought against him. ... He's pleaded not guilty. He is not guilty."
Authorities in Boston are working with police in Warwick, Rhode Island, on what could be a related case. On April 16 at a Holiday Inn Express in Warwick, a man tied up and demanded money from a 26-year-old dancer who had posted a Craigslist advertisement, Warwick Police Chief Col. Stephen McCartney said.
The robbery was interrupted when the woman's husband entered the room. After pointing his gun at the husband, the suspect fled, McCartney said. He said the incident "may be related to similar crimes occurring in the Boston area," but that had not been determined.
Craigslist CEO Jim Buckmaster told CNN the company is "horrified and deeply saddened that our community services have been associated in any way whatsoever with a crime of violence." He promised that Craigslist will evaluate the incident to see whether additional measures could be introduced to protect users.
CNN's Mary Snow and Randi Kaye contributed to this report.