YAPHANK, N.Y. — Four young women who police believe were slain by a serial killer, including a mother from Scarborough, all worked as prostitutes and advertised online, investigators said Monday.

Investigators did not identify a suspect or say how the women were killed, but they were looking into what clients the victims might have met shortly before they disappeared. One of the women was reported missing nearly 3½ years ago; another was seen as recently as September.

The four bodies were found in early December, strewn along a quarter-mile of marshy grassland on a beachfront strip on Long Island.

Last week, authorities identified one of the victims as Megan Waterman of Scarborough, who advertised on Craigslist as an escort. She was 22 when she disappeared during a stay in Hauppauge, N.Y., on the weekend of June 5.

On Monday, the other women were identified by law enforcement as Maureen Brainard-Barnes of Norwich, Conn., Melissa Barthelemy of Erie County, N.Y., and Amber Lynn Costello of North Babylon, on Long Island.

Brainard-Barnes was last seen in Manhattan on July 9, 2007; Barthelemy reportedly was living in the Bronx when she disappeared on July 12, 2009, police said. Costello was last seen in North Babylon on Sept. 2.

Brainard-Barnes was 25 when she disappeared; Barthelemy was 24 and Costello was 27.

District Attorney Thomas Spota said he believes the women were victims of a serial killer. The bodies “probably were in that location for some period of time,” he said. Investigators believe the women were killed elsewhere and their bodies were dumped along the beach highway.

Police Commissioner Richard Dormer, who had backed away from early assertions that a serial killer was involved, agreed with Spota.

“What activities these victims may have engaged in prior to their murders does not matter,” Dormer said Monday. “They were young women whose lives were cut tragically short.”

Spota said “the actual cause of deaths (appears) to be substantially similar” and “it appears the same person or persons are responsible.” He said he would appeal to women who work in the escort business to come forward if they have details about the slain women.

Waterman disappeared after going to New York on Memorial Day weekend with Akeem Cruz, a 21-year-old Brooklyn man described as her boyfriend. Cruz is now serving a 20-month sentence in the Maine Correctional Center for drug trafficking.

Police found the first body on Dec. 11 just steps from the Ocean Parkway, which leads to the popular Jones Beach State Park. The other three bodies, including Waterman’s, were found two days later during a follow-up investigation.

Police were looking for another missing Craigslist escort when they happened upon the bodies near the beach. They have since said that person, a woman from New Jersey, was not among the bodies recovered there. That case is still under investigation, authorities said Monday.