Suffolk County District Attorney's Office spokesman Jake Wark said that Boston investigators hope the ad, posted in the "erotic services" section of the Web site, will persuade other possible victims to come forth.
"Were you attacked or robbed at a Boston-area hotel after placing an ad on Craigslist?" the ad asked. "If so, you may have information that could aid the investigation into the April 10 armed robbery of a woman at the Westin Copley Place Hotel and the April 14 murder of a woman at the Marriott Copley Place Hotel. Both victims were attacked by a prospective client who had contacted them through ads placed on Craigslist."
The posting does not mention Markoff by name, and Wark says investigators aren't sure there are other victims.
Markoff, a Boston University medical student, is charged in the April 14 killing of Julissa Brisman, a 25-year-old masseuse he met through Craigslist. He is also charged in a robbery of another masseuse that police say he met through Craigslist.
Also Thursday, a law enforcement official, speaking to The Associated Press on the condition of anonymity because he was not authorized to discuss the case, said Markoff had been placed on suicide watch at the Boston jail where he is being held. ABC News and The Boston Globe, citing unidentified law enforcement sources, reported that what appeared to be shoelace marks were found on his neck.
Defense attorney John Salsberg told reporters he was concerned about Markoff's well-being but otherwise declined to comment on the reports.
Jail officials declined to comment to The Associated Press, citing privacy rules.
In an exclusive interview with the New York Daily News, a college friend of alleged "Craigslist Killer" Philip Markoff discussed an incident with him four years ago - possibly fueled by alcohol - which may have provided a glimpse into the medical student's dark side, reports CBS News correspondent Daniel Sieberg.
"He pushed me up against the wall and tried kissing me," Morgan Houston told the paper in the videotaped interview, obtained exclusively by CBS News.
"I couldn't physically get him off of me," Houston told the paper.
Investigators were not certain the Craigslist ad would produce new leads.
"There may not be any other victims out there, but if there are and they have not seen or are not comfortable responding to our requests in the mainstream media, this may be a way to make contact with them," Wark said.
Markoff has pleaded not guilty to charges of murder, armed robbery and kidnapping.
Meanwhile, supporters of Markoff have launched a new Facebook group to rally around him.
The new Facebook group called "Phil Markoff is Innocent Until Proven Guilty" describes itself as a forum to rally against the media and a culture "that has forgotten that people like Phil are suspects, not killers."
Markoff was arrested Monday in Walpole, less than 20 miles southwest of Boston, as he drove with his fiancee, Megan McAllister, to Foxwoods Resort Casino in Connecticut.
McAllister, who earlier this week said Markoff would not hurt anyone, is standing behind him, her father, James McAllister, told reporters Thursday.
"She's still confident in Phil. Other than that, we're saying a lot of prayers," he said outside his home in Little Silver, N.J.