YAPHANK, N.Y. — Investigators returned today to their theory that four young women were slain by a serial killer and dumped on a desolate stretch of barrier island, as authorities said all the bodies were identified as those of prostitutes who advertised online.

Officials on New York’s Long Island, where the bodies – including that of Megan Waterman of Scarborough, Maine – were found in early December strewn along a quarter-mile stretch of marshy grasslands in overgrown shrubbery and sea grass, would not say today whether they had identified any suspects.

Three women were identified today by county law enforcement as Maureen Brainard-Barnes, of Norwich, Conn., Melissa Barthelemy, of New York’s Erie County, and Amber Lynn Costello, of North Babylon, on Long Island. Authorities said those locations were where they were last seen, not necessarily where they lived.

Authorities last week identified the first of the four women as that of Waterman, a 22-year-old Craigslist escort, who disappeared June 6.

District Attorney Thomas Spota said that he believes they are victims of a serial killer. The bodies “were probably were in that location for some period of time,” he said.

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 today. “They were young women whose lives were cut tragically short.”

Spota said he would reach out to women who work in the escort business, appealing to them to come forward if they have details about the slain women.

Brainard-Barnes was 25 when last seen in Manhattan on July 9, 2007. She had left Connecticut for New York that day and never returned, according to several missing-person websites.

Barthelemy was 24 when she disappeared July 12, 2009, and Costello was 27 when last seen Sept. 2, 2010. A woman matching Costello’s name, birth date and appearance failed to appear in February in court in Pinellas County, Fla., on theft charges and was briefly jailed there twice in 2009 for shoplifting.

Waterman was last seen at a Holiday Inn Express in Hauppauge on Long Island. She disappeared after traveling 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 at the Maine Correctional Center for drug trafficking.

Police found the first body Dec. 11 just steps from Ocean Parkway, which leads to the popular Jones Beach State Park. The other three bodies, including that of the missing Maine woman, 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 the person they were originally looking for, a woman from New Jersey, was not among the dead. Authorities said today that case is still under investigation.

The highway sits on a narrow strip of land that divides the Great South Bay from the Atlantic Ocean. The four-lane parkway runs through the middle, connecting Jones Beach with several state- and town-run beaches to its east.

2:21 p.m.

YAPHANK, N.Y. — Suffolk County authorities have identified the remaining three bodies found near a parkway leading to Jones Beach on Long Island.

Officials say the bodies were identified as three women who worked as prostitutes or escorts and advertised their services online.

They were identified as Maureen Brainard-Barnes of Norwich, Conn.; Melissa Barthelemey, of the Bronx; and Amber Lynn Costello, of North Babylon, on Long Island.

Last week, authorities identified the first of the four women as that of Megan Waterman, the Scarborough, Maine, woman who had been missing since last May.

District Attorney Thomas Spota said today the deaths may be the work of a serial killer. Initially Suffolk County police thought so, but then had backed away from the theory.

Officials say the women were believed to be killed elsewhere and dumped along the stretch.

11:36 a.m.

YAPHANK, N.Y. — Suffolk County police and the district attorney say there are new developments in the investigation into four women’s bodies found near a parkway leading to Jones Beach on Long Island.

Last week, authorities identified one of the four women as that of Megan Waterman, who was from Scarborough.

A spokesman did not immediately have details on the developments. A press conference is planned for this afternoon.

District Attorney Thomas Spota said at the time that authorities were close to identifying the other three bodies.

Police Commissioner Richard Dormer had initially indicated a serial killer might be responsible for the deaths. However, police have since backed away from that assertion. They now say they’re not certain how the women died.