Items from two attack victims were found in accused "Craigslist Killer" Philip Markoff's Boston-area apartment, a law enforcement source said.
A law enforcement official who spoke on the condition of anonymity told The Associated Press that police found belongings of both women in Markoff's apartment in Quincy, south of Boston.
The source wasn't authorized to speak about evidence in the investigation.
The official would not confirm reports by the Boston Herald and ABC News that the items seized by police included the victims' underwear and that he appeared to be collecting the lingerie as "souvenirs."
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The news came as more details emerged Wednesday about the case authorities are building against the 23-year-old Boston University medical student.
Markoff has been charged with targeting women who advertised erotic services on Craigslist, luring them into hotels to rob them and killing one woman who fought back.
A Connecticut casino confirmed that investigators are looking into gambling as a possible motive in the crime spree.
Markoff was a "frequent visitor" to Foxwoods casino in Connecticut, a law enforcement official said. He was arrested on Monday on Interstate 95 in Massachusetts as he and his fiancee drove to Foxwoods Resort Casino.
"We do not release information publicly concerning individual patrons; however, we are aware of this situation and are cooperating with the appropriate law enforcement authorities," the casino and the Mashantucket Pequot Tribal Nation said in a statement.
Suffolk District Attorney Daniel Conley refused to comment directly on reports that Markoff might have been seeking money to pay gambling debts.
"As we explore various motives for the robberies, gambling is certainly a motive that we will look at," he said.
Markoff is charged with murder in the April 14 shooting death of 26-year-old Julissa Brisman of New York City at a luxury hotel in Boston and with kidnapping and armed robbery in the other assault on April 10 at a different hotel in the city.
A former medical school lab partner of Markoff's said he was prone to profound mood swings and seemed "disturbed."
Tiffany Montgomery told The Boston Globe that she's "not even remotely surprised" Markoff is a suspect in the case and said she was often alarmed at his extreme changes in mood.
Markoff would appear warm and friendly one day and then brooding and depressed the next, according to Montgomery.
She said she was so troubled by his behavior that she considered alerting Boston University school counselors that he might be suicidal.
"He just wasn't right in the head, and I knew it, and probably other people did, too," Montgomery, 26, told the Globe.
The two spent hours together in the lab while they were students at Boston University School of Medicine, according to Montgomery, but they never became close.
Markoff spoke only when someone else initiated a conversation, Montgomery said, adding that he seemed nice but "strange in a dark way."
A former high school classmate of Markoff's told the Boston Herald that he was a contemptuous high school bully.
"He had this hatred for people he thought were beneath him," said the classmate, Caitlin, 23, who asked that her last name be withheld. "He saw me as beneath him and he needed to degrade me. I think he had a complex."
On Tuesday, the med student — who was engaged to be married this summer — was ordered held without bail in Boston Tuesday on charges he used Craigslist to lure a masseuse to a hotel room and then shot and killed her.
Prosecutors say they found a semiautomatic weapon at his home, as well as ammunition and materials exactly like those used in an attack on another masseuse in Boston.
Authorities said they followed a computer trail to Markoff, linking an account used to set up appointments on Craigslist with the two women who were attacked to his address in Quincy.
The suspect's fiancee says police have the wrong man and there is no way he could have done it.
In an e-mail to ABC, Megan McAllister said Markoff "could not hurt a fly." She called him "a beautiful person inside and out," according to the message read on the air Tuesday.
The wedding was planned for August, McAllister said.
Investigators believe he could be involved in a slew of similar cases and have themselves posted ads on Craigslist in the hopes of finding other women who might have been attacked by Markoff.
"There may be other victims out there, and if you are, we want you to come forward," Conley said Tuesday. "He's a predator."
Detectives said they had been tracking Markoff for some time before arresting him Monday. Markoff was seen on video surveillance using a hand-held texting device at several of the hotels, according to police.
Brisman was found dead April 14 at the Boston Marriott Copley Place. The New York City woman had a massage table set up in the room where she was shot.
Police said they had long suspected that Brisman's killer was connected to the robbery of another woman who advertised on Craigslist at the Westin Copley Hotel in Boston four days earlier.
Craiglist CEO Jim Buckmaster on Wednesday defended the Web site, saying is an "extremely unsafe venue for criminal activity because you're virtually guaranteeing that you're going to get caught."
"That's been the case with nearly every serious violent crime that's been connected with the site," Buckmaster said in a telephone interview from San Francisco, where Craiglist is based. "There's an electronic trail leading to yourself. So don't use Craigslist for crime unless you want to go to jail."
Authorities believe Markoff also may be linked to an attempted robbery in a Rhode Island hotel Thursday 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 Facebook profile lists Markoff as a 2007 graduate of State University of New York-Albany and in the 2011 class at Boston University. Markoff was suspended from medical school after his arrest.
The Associated Press contributed to this report.