In a life controlled by heroin addiction and prostitution, Amber Costello stuck by a rule of self-preservation, her boyfriend and a roommate said: Don't get into a john's car.

On Sept. 2, Costello abandoned that precaution and left her West Babylon home in a client's vehicle, they said. She took nothing with her -- no cellphone or purse -- and told no one where she was going.

Three months later, Costello's remains were found 18 miles away, wrapped in burlap and hidden in the thick brush off Ocean Parkway near Gilgo Beach. Nearby were remains of three other women who worked as prostitutes -- Maureen Brainard-Barnes, 25, Melissa Barthelemy, 24, and Megan Waterman, 22. Their deaths are likely the work of a serial killer or killers, Suffolk authorities said.

Like Costello, who was 27, the women usually drew their own safety boundaries in a risky business, friends and family said. But just before their deaths, three did something out of character: Costello and Waterman both accepted "car dates," law enforcement sources said, and Brainard-Barnes apparently strayed from her normal workplace -- New York City hotels.

Less is known about Barthelemy's case, although she, too, made a change in the months before her July 2009 disappearance. After disputes over money with her pimp, she had begun secretly advertising her services online and working independently, said her mother, Lynn Barthelemy.

Why the women took steps outside their normal routine remains a mystery to those who knew and loved them. Were they offered more money for that final date? Was he someone they knew? Was he a repeat customer they trusted?

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'She said it was big money'

Suffolk investigators would not characterize the victims' actions before their fatal encounters, nor comment on their significance. A law enforcement source said detectives are focused more on their final movements and possible interactions with the killer than their broader behavior patterns.

Mary Ellen O'Toole, a former FBI profiler who has worked on serial homicide cases, said the women's work was so dangerous that no amount of preparation could have ensured their complete safety. But she said the Gilgo killer may have persuaded the victims to let down their guard.

"However he projected himself to them, they may not have seen any inherent dangers," O'Toole said.

The last night she was seen alive, Costello had two lengthy conversations with a man who answered her Craigslist Long Island ad, said her roommate, David Schaller. She agreed to meet him in a car parked around the corner, Schaller said. The client would pay $450 for two hours or $1,500 if she spent the night with him.

But the man set conditions: She was to bring nothing and tell no one where she went.

Schaller told her not to go.

"You can't do this," he said he told her. "At least take a phone so you can call me if something happens."

Costello refused, he said. "That's not what he wants," Schaller remembers her saying. "She said it was big money."

It's unclear why, but "something made her trust him," recalled Schaller, who said his cellphone was used for the conversation and that he overheard parts of it. "It was like she knew him."

Abandoning their own rules

Crystal DeBoise, co-director of the nonprofit Sex Workers Project in Manhattan, said most prostitutes -- from street walkers to high-end call girls -- develop safety rules. Many work only in hotels, others inform friends and pimps of their whereabouts and some even ask clients for references.

"Are there people who can't follow safety precautions all of the time? Yes, just like any industry, sometimes those rules are abandoned," DeBoise said.

Brainard-Barnes' sister still wonders how she ended up on Long Island. "My sister said she would never go outside the five boroughs," said Melissa Cann, 27, of New London, Conn. "She would only do in-calls at hotels where she knew that the front desk had security and video cameras."

Brainard-Barnes, a single mother from Norwich, Conn., struggling to support two children, was introduced to prostitution through a modeling job in Manhattan, her sister said.

To her family, it was an unexpected path for a bookish young woman who published poetry on her MySpace profile and invented games for her children. Brainard-Barnes insisted to her sister that it was not her full-time profession and in early 2007 she worked as a telemarketer.

By July, however, Brainard-Barnes had been laid off and faced eviction from her apartment, Cann said. On Friday, July 6, 2007, Brainard-Barnes left her children with their fathers, took a train to Manhattan, got a hotel room near Times Square and posted an ad on Craigslist, Cann said.

Brainard-Barnes did not return to Connecticut the following Monday, as she had promised, Cann said. She did call friends that day -- her last contacts before going missing. "There was nothing distressful," Cann said.

The next year, police contacted Cann and asked whether her sister had ever worked on Long Island. "Never," Cann replied.

Police then told Cann a surprising detail: Her last cellphone call pinged a tower on the South Shore, Cann said.

Wanted to open hair salon

Melissa Barthelemy had already had a brush with prostitution's dangers. A john once tried to mug her near her Bronx home with a knife. Barthelemy, who stood 4-foot-11 and weighed 95 pounds, grabbed the weapon and stabbed her attacker, her mother said.

"She was always tough," Lynn Barthelemy said.

The Buffalo native moved to New York City in 2006 to make money to start a hair salon. A man convinced her prostitution offered quick cash and acted as her pimp. But the relationship was turbulent, Lynn Barthelemy said. After her disappearance, her mother found a letter Melissa wrote to him.

"It basically said that she was sick of giving her money up," Lynn Barthelemy said.

Barthelemy began posting Craigslist ads without the pimp's knowledge, her mother said. He found out, and an altercation ensued, she said.

Her last known phone call went to the pimp at 11:58 p.m. on July 10, 2009, but it appears from its short length -- less than a minute -- that he didn't answer, her mother said. She was last seen the afternoon of July 12, sitting on the curb outside her Bronx apartment.

Date hidden from pimp

Waterman's family was stunned when they learned the high school dropout from Scarborough, Maine, was working as a prostitute. Her maternal grandmother, Muriel Benner, who raised her and her brother Greg, warned of dangers, but Waterman replied she and her boyfriend, Akeem "Vybe" Cruz, had things under control.

"Akeem won't let anything happen to me," Waterman said, according to Benner. She told her aunt, Elizabeth Meserve, that Cruz was in a nearby room during each client session.

Nicole Haycock, 22, who said Waterman was her best friend, believes selling sex had a dual appeal for Waterman: love for Cruz, whom she wanted to please, and money for her daughter Liliana, then 3 years old. Liliana is now being cared for by family.

Suffolk cops say Cruz, serving a 20-month sentence in a Maine prison on a drug trafficking conviction, acted as Waterman's pimp. An FBI search warrant for Cruz's laptop says he is being investigated for prostitution-related crimes and drug trafficking. Robert Napolitano, Cruz's attorney in Portland, Maine, didn't return calls for comment.

According to her family and the warrant, Waterman saw clients in hotels. She did not, family and friends said, get in clients' cars or go to their homes.

Cruz and Waterman left Portland by bus on June 1, bound for Manhattan, police and family said. Waterman checked into a room at the Holiday Inn Express in Hauppauge. Cruz uploaded Craigslist ads, the warrant said, with a photo and cellphone number of a woman her family says is Waterman.

On June 4, Waterman called Haycock with some news: Cruz wanted her to have his child and she would no longer have to work as a prostitute. "I never heard her as happy as she was that day," Haycock said.

Her family finds what happened the next night puzzling.

At 8 p.m. June 5, the hotel security camera captured Cruz and Waterman leaving the hotel together, said Waterman's mother, Lorraine Ela. At 8:30, Waterman entered the hotel alone, Ela said.

Around 1:15 a.m., Waterman got a call from Cruz, Ela said phone records show. It's not known what if anything was said between them. Security video shows Waterman leaving the hotel at 1:30, police said. Ela said she has seen the video, which showed Waterman was alone.

A law enforcement source said she had arranged a date without Cruz's knowledge and met the man in his car.

At 11 a.m. on June 6, Cruz called Benner, asking about Waterman's whereabouts.

"She was supposed to be with you?" Benner said she responded. "Where is Megan?"

Money fueled drug habit

Amber Costello's rule about staying out of a john's car came after a brutal lesson.

Her roommate Schaller, who helped her with clients, said he could recall her getting into a man's car only once -- after their session was over and he offered her a ride to a convenience store.

"She called me from the side of the Southern State Parkway. Her head was bleeding," Schaller recalled. "He had punched her three times in the face and left her on the side of the road."

Costello told her boyfriend, Bjorn Brodsky, 26, that sex work had been part of her life since she was 17. She was also a heroin addict, according to Schaller, Brodsky and her relatives.

A Wilmington, N.C., native, Costello did not graduate high school, married and divorced twice and bounced around jobs in North Carolina and Florida before moving to New York in late 2009. Schaller, a friend of Costello's sister Kimberly Overstreet and then a commercial fisherman, said he paid for her plane ticket.

She attended a 28-day rehab program at St. Charles Hospital in Port Jefferson, where Brodsky said he met her. Both of them relapsed into drug use by the following spring.

Costello bought a prepaid cellphone and uploaded a Craigslist post advertising herself as a private dancer named Carolina, Brodsky said.

"That phone never stopped ringing," he said.

Schaller allowed her to work from his home. He and Brodsky say they did not act as pimps for Costello. But one of them was usually in another room when she had clients over. There were rules: Clients came to her and paid $200 an hour; she went only as far as she wanted to; and if a client demanded more, she called out for Schaller or Brodsky. The men would make the client leave -- in some cases physically removing him.

"It was a very controlled environment," Schaller said.

By midsummer, Brodsky said, Costello was earning $7,000 a week.

"She would lay the cash down on the table and say, 'Here, you can have $3,000 and I'll take four,' " Brodsky said.

By August, Brodsky was back in rehab. Costello kept living with Schaller until her disappearance the next month. Her vanishing didn't immediately cause alarm, since she had left at other times as well.

But in December, when Costello was finally reported missing by her sister after remains were discovered at Gilgo, a homicide detective called Schaller. The investigator wanted details about her clients, especially regulars, and the last caller who persuaded her to go for a ride.

"It had to be somebody she was comfortable with," Schaller said. "Or maybe all that money just got her in the head."

With Andrew Strickler and Anthony M. DeStefano