Philip Markoff said nothing during the brief hearing in Boston Municipal Court.
Authorities said they followed a computer trail to Markoff, linking an account used to set up appointments on Craigslist with two women who were attacked to his address in Quincy.
They say a search of his home found the gun, ammunition and materials exactly matching those used in an attack on another masseuse in a Boston hotel.
Markoff is charged with murder in the death of Julissa Brisman of New York City and kidnapping and armed robbery in the other assault.
The second-year Boston University medical student lives with his fiance, who claims police have the wrong man.
"He could not hurt a fly," Megan McAllister said in an e-mail to ABC's "Good Morning America. "All I have to say is Philip is a beautiful person, inside and out," she said in the e-mail read on Tuesday's program.
Markoff's attorney, John Salsberg, did not immediately comment.
Authorities have said there could be more victims.
"Our top priority is holding Philip Markoff accountable. He's a predator," Suffolk District Attorney Daniel Conley said Monday night in announcing Markoff's arrest earlier in the day during a traffic stop south of Boston.
An e-mail message sent to McAllister via her Facebook page was not immediately returned Tuesday. The McAllister family did not respond to knocks of the door of their Little Silver, N.J. home Tuesday. Shortly after, a police officer emerged from home and said the family did not wish to speak with reporters, and asked the media respect their privacy.
Michael Bernard, who lives in Markoff's building in Quincy, was shocked to recognize his neighbor on television reports of the killing.
"He was smart, he carried himself well, he was clean, a good looking guy," said Bernard, a retired electric company worker. "He seemed like the type that would have it all. It doesn't make sense."
Authorities believe Markoff also may be connected to the attempted robbery Thursday in Warwick, R.I., of a stripper who had posted an ad on Craigslist. She was held at gunpoint before her husband entered the room and her attacker fled.
A Boston University spokeswoman said Markoff was suspended from the medical school after his arrest.
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