A law enforcement official who spoke on the condition of anonymity said police found items belonging to both women in Markoff's apartment in Quincy, south of Boston. The source wasn't authorized to speak about evidence in the investigation.
The source would not confirm a report by ABC News that the items seized by police included the victims' underwear. ABC News said two unidentified law enforcement sources said Markoff appeared to be collecting underwear as "souvenirs" from his alleged victims.
ABC News did not say how Markoff took the underwear, and he has not been accused of sexually assaulting either woman. Prosecutors have said he went through the purse of the first woman he allegedly robbed.
Investigators are looking into gambling as the possible motivation. The law enforcement source said Markoff was a "frequent visitor" to Foxwoods casino in Connecticut.
Markoff, 23, is accused in the death of Julissa Brisman, 26, of New York City, a masseuse who was found dead in the doorway of her hotel room after being bashed in the head and shot three times. He's also accused of robbing and tying up another woman, and police have said there could be more victims.
Foxwoods Resort Casino, in Ledyard, Conn., has confirmed it is cooperating with authorities investigating Markoff's gambling habits.
Markoff was arrested on Monday on Interstate 95 in Massachusetts as he and his fiancee drove to Foxwoods.
Suffolk District Attorney Daniel Conley said he believes Markoff's original motive was robbery, but he ended up killing Brisman when she fought back.
The first woman Markoff is accused of robbing said she believes she's alive because she didn't resist.
"I just complied with everything he wanted me to do and I didn't resist him in any way and I think that's why," she said in an interview with Boston television station WCVB. "I just hope that they can put him behind bars for the rest of his life."
Prosecutors have not released her name.
The woman, 29, of Las Vegas, who also advertised as a masseuse on Craigslist, was allegedly attacked on April 10 at the Westin Copley Hotel.
She said she had identified Markoff from the surveillance photos police distributed. He was wearing the same clothing and she recognized his face, she said.
The woman said she was able to slip out of the plastic ties she was bound with about a minute after Markoff left and escape. She said she was robbed of $800 in cash, a debit card and $250 in American Express gift cards.
Markoff is also suspected in an attempted robbery in Warwick, R.I., of a woman who had posted a Craiglist ad as a stripper. She was held at gunpoint before her husband entered the hotel room and her attacker fled.
Rhode Island Attorney General Patrick Lynch said in a statement Wednesday that Warwick police have developed "promising information" about the April 16 robbery at the Holiday Inn Express & Suites.
"Although we're encouraged by the progress being made, this is a complex investigation and is going to take more time," Lynch said.
Connecticut Attorney General Richard Blumenthal on Wednesday called on Craigslist to stop what he calls "pimping and prostitution in plain sight." He asked the site to immediately eliminate photographs in the "erotic services" section, hire staff to screen images and ads that violate the site's terms of service and fine those who violate those terms.
Craigslist representatives did not immediately return a call seeking comment.
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