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?