GARDEN CITY, N.Y. – The four women, including a mother from Maine, were prostitutes in their 20s, advertising their services online, living on society’s margins and not likely to be immediately missed. They appeared to share similar middle-class childhoods, but the way they lived their adult lives is what has police chasing a serial killer.

The bodies were found in mid-December, strewn 500 yards apart along a deserted barrier island highway off New York’s Long Island. Suffolk County police have classified their deaths as homicides, but no suspects have been publicly identified and officials have refused to reveal how they were killed.

Once investigators determined they were all sex workers, though, it wasn’t a leap to say they had a killer in common.

Prostitutes may often work alone and have few community ties, making them in many ways a serial killer’s more likely victims. Gary Ridgway, the Seattle-area “Green River Killer” of the 1980s and ’90s, preyed on prostitutes. So have many other notable serial killers, including Jack the Ripper, who terrorized London in the 1880s.

“The abduction is very easy. It’s not like breaking into a house and kidnapping someone,” said Louis B. Schlesinger, professor of forensic psychology at the John Jay College of Criminal Justice in New York. “And in many instances, police don’t know the person is missing for a very long time afterward.”

In Atlantic City, detectives have yet to capture the person responsible for the deaths of four prostitutes found in 2006. And in Daytona Beach, Fla., police have been investigating the deaths of four women found slain in 2005 and 2006. Those women led “high-risk lifestyles” — they were prostitutes or used drugs, and were likely felled by a serial killer, police said.

The women found dead on Long Island grew up in Maine, Connecticut, North Carolina and upstate New York. One was a promising hairstylist. Two others were described as loving mothers; one called home three times a day to speak to her 4-year-old daughter.

An ex-husband of 27-year-old Amber Lynn Costello, originally of Wilmington, N.C., said their marriage ended when they argued over her heroin use.

“I told her to hit the road,” said Michael W. Wilhelm, 41, of Kannapolis, N.C.

Costello was the most recent victim to be seen alive; police say she was last seen in September living in North Babylon, several miles north of where her body was found.

Relatives knew that 22-year-old Megan Waterman of Scarborough, Maine, was advertising her escort services online.

Waterman, mother of the 4-year-old, was last seen at a Hauppauge, N.Y., hotel in June 2010. She had apparently traveled to New York on Memorial Day weekend with Akeem Cruz, a 21-year-old Brooklyn man described as her boyfriend. He is serving a 20-month sentence for drug trafficking in the Maine Correctional Center.

Maureen Brainard-Barnes, also a mother, was the first of the four women to vanish. She left Norwich, Conn., on July 9, 2007, and headed for New York City, planning to return the following day. A Norwich Police Department missing-persons flier notes there are few details about her disappearance.

“When she stopped calling people, everybody knew something was wrong,” said Sarah Marquis of Groton, Conn., who described herself as a close friend.

Melissa Barthelemy grew up in Buffalo, N.Y., and trained as a hairstylist. After moving to New York in 2007, she turned to prostitution — to supplement her income, Barthelemey family attorney Steven Cohen said.

“Nobody goes down to New York City to become a prostitute, an escort,” he said. “She did this because she was behind on rent from time to time.”

Her mother thought she was working as an exotic dancer.

“I had told her I wasn’t too happy about it,” Lynn Barthelemy said. “But I told her to be careful. She said, ‘Mom, I’m never alone, I always have a friend and it’s just dancing.’ “

Police said all four advertised on Craigslist or other online services. Craigslist, which is largely free, last month said it had removed its “adult services” section from both its domestic and its international sites.

Prostitutes who work on the streets are the most vulnerable to violence, but “indoor sex workers,” including call girls and so-called Craigslist escorts, also contend with dangers, said Sienna Baskin, co-director of the Urban Justice Center’s Sex Workers Project, which provides advocacy and legal services.

“The sex workers are aware of the risks, and there’s a lot of shared knowledge on how to reduce the risks,” Baskin said. “It can be anything from using condoms to screening clients, asking for references and choosing where they work, where they feel safest.”