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Long Island – from www.nytimes.com – A chill has swept through the region’s underground sex trade.

New clients have grown skittish. Earnings have shrunk. In April, a prostitute visiting from Chicago arrived in New York City, unaware that a serial killer going after prostitutes appeared to be on the loose on Long Island. Upon hearing the news, she was so unnerved that she cut short her trip and booked a return flight the following day.

For prostitutes in the New York City area, the murders of four of their fellow workers, all of whom disappeared between 2007 to 2010, have intensified the fear and distrust that is always attendant to their still-criminalized line of work. That uneasiness has only increased this month, as the authorities disclosed the identify of a fifth victim left along Ocean Parkway on Jones Beach Island, and revealed that, like the other four, she had been a prostitute.

Pimps have exploited the terror to their advantage, using it to reinforce their control over their female workers. One 23-year-old prostitute recalled how her pimp warned her that the Long Island victims had made the mistake of working independently and “didn’t tell their daddy where they were.”

Meanwhile, that woman’s father was imploring her to leave the prostitution business and go back to school. He sent her a text message earlier this year saying, “I don’t want you to end up like those girls in Long Island,” the woman, who insisted on anonymity, recalled during a recent interview in a cafe in Lower Manhattan.

The discovery of bodies on Long Island has cast a light on the illicit sex trade, forcing many women to rethink what they do and broaden whatever precautions they may take. The killings have brought further changes to a field that has taken a traditional streetside transaction and expanded it to the point where it can now occur anywhere, and include anyone with an Internet connection.

Of the five prostitutes identified on Jones Beach Island, only one lived on Long Island. Four of them advertised on Web sites, arranging meetings from places as distant as Maine.

The woman from Chicago, for example, intended to stay in New York for a week, advising potential clients in her online advertisement that she would be splitting her time between Queens and Manhattan.

Prostitutes who work on Long Island now find themselves playing amateur detective, re-examining any experiences with customers that stood out for being particularly off-putting or threatening.

Past cases have demonstrated that serial killers who single out prostitutes “don’t kill everyone they come into contact with,” said Mary Ellen O’Toole, a former agent with the Federal Bureau of Investigation who has specialized in profiling serial killers for the bureau.

Some local prostitutes have probably survived encounters with the killer, she said, some without even knowing the grave danger they were in while in the killer’s company.

“There will be women, they don’t even know they’re survivors,” Ms. O’Toole said.

Ms. O’Toole said investigators could find clues by focusing on local sex workers as a potential victim pool. Among the questions for those workers is whether any of their clients’ emotions flipped instantly between anger and arousal, in what she termed “Jekyll-Hyde switches.”

“I have a couple of cuckoo clients,” said one prostitute who advertises online but declined to provide her name out of fear of being arrested.

“Everyone has a couple of clients that are kind of crazy and we all wonder if one of them is him. If 20 women identified a really whacked-out stalker type, then there could be reason to put surveillance on him.”

But the police, she said, “don’t make it comfortable to phone in a tip.”

Organizers of sex workers, many of whom advocate decriminalizing prostitution, say prostitutes should be given blanket amnesty or immunity in exchange for cooperating with law enforcement.

“There’s petitions calling for amnesty and we support that call,” said Sienna G. Baskin [pictured], a lawyer and co-director of the Urban Justice Center’s Sex Workers Project.

Ms. Baskin said the police might consider ceasing vice raids and one-on-one stings, “while investigating this more serious crime, to build better relations between sex workers and the police.”

The Suffolk County police commissioner, whose department is leading one of the area’s largest multiagency homicide investigations in years, said some of the leads it was pursuing had been provided by people in the sex trade.

“Detectives have interviewed people in the escort business regarding the case, and they will continue to be interviewed,” the commissioner, Richard Dormer, said in a statement.

Escorts who have come forward to volunteer information have not been charged with a crime, Mr. Dormer said, adding that the policy would remain in place for any prostitutes who offer information.

But Kate Mogulescu, a lawyer for the Legal Aid Society, said she had recently asked four new female clients whether New York City police officers had debriefed them about the Long Island murders, either before or after arresting them for prostitution. Each of the women told her that the arresting officer or detective had not inquired about the case, said Ms. Mogulescu, who defends people accused of prostitution in Manhattan.

Of course, whether prostitutes would be forthcoming in such a situation is another question.

After the 23-year-old prostitute was raped at gunpoint last December, she lied to the police, telling them that her attacker had presented himself as a Samaritan who had pulled over to help when she ran out of gas. She did not want to admit that she was working the streets that night, even though she was attacked in an area known for prostitution, and her appearance — “I was dressed like a hooker,” she said — probably gave her away.

“Whether they are out there to protect you, or out there to lock you up, you didn’t want to find out,” the woman said during the interview, accompanied by her lawyer, Ms. Mogulescu. The woman has recently left prostitution, and said she hoped never to return.

Prostitutes say they must rely on safety protocols tailored for specific locations, from suburban hotels to the streets of Hunts Point, in the Bronx, or East New York, in Brooklyn.

The 23-year-old worked the streets of the Bronx and Brooklyn and advertised her services online, blurring the distinction between streetwalker and Internet escort.

She said she had different safety protocols for each environment; in Hunts Point she would never allow a client to drive her more than three blocks from their meeting point, to stay in a zone where people knew to look for her. She and another worker would sometimes send their pimp a text message containing the license plate of the vehicle she was entering when accepting a date.

When setting up dates in hotels, she said, she would purposely send the client to the front desk, so the clerk would be in a position to identify the face of the man going to her room if anything went wrong later. During most dates, even those arranged online, she surreptitiously called her pimp on her cellphone and left the speakerphone setting on so he could listen in for the duration of the tryst.

Advocates for sex workers have long encouraged prostitutes to evaluate their clients thoroughly.

Another prostitute who said she worked as an escort in New York said that with the possibility of a killer on the loose, some “are not only asking for personal information, but they are asking for references of sex workers the client has visited.”

Such rigorous vetting is not always possible for many prostitutes.

Asked what her deal-breakers with a client were, the 23-year-old cited two: drunkenness and an empty wallet.

“Other girls look for drunks, to rob them,” she said. “My pimp wanted me to do that, and was angry that I wouldn’t rob.”