Another Craigslist user has died during an encounter arranged through the site, and police believe the perpetrator may still be using the site to search for more victims.

Boston police are investigating the slaying of Julissa Brisman, a 26-year-old from New York City, who had advertised massage services on Craigslist. Brisman's friends have told local and national media that the young woman earned money through a variety of means, including attending parties partially dressed and flirting with men. A massage table was found in the Boston hotel room where Brisman died Tuesday during what police believe was a robbery attempt. Police are investigating the possibility that she worked for an escort service.

"There appears to be a psychopath on the loose in the Boston area, and we will of course do everything in our power to assist law enforcement in apprehending the perpetrator as soon as possible," Craigslist CEO Jim Buckmaster said Friday. "Our hearts go out to the family and friends of Julissa Brisman. We are horrified and deeply saddened that our community services have been associated in any way whatsoever with a crime of violence."

Many ads on Craigslist for romance and massages are covers for prostitution, but it's still unclear exactly what type of transaction the victim had arranged. Whether or not the victim and killer set up a meeting to exchange money for sex, the crime is one of a series in which Craigslist users have died after perpetrators lured victims through the site.

Police are trying to determine whether the killing is connected to a robbery of a woman offering private dances in Rhode Island last night and a robbery of a woman who met a man in a Boston hotel last week after advertising erotic services.

Police across the United States have investigated and cracked down on countless crimes that resulted from in-person meetings among those who begin communicating through the site. The crimes aren't all related to ads for sex.

Michael Anderson of Minnesota was sentenced earlier this month to life in prison without the possibility of parole for the shooting death of Katherine Olson, a 24-year-old who answered a Craigslist ad for baby-sitting in October 2007. Buckmaster said that relatively few crimes take place among the people meeting through Craigslist.

"Given the crime rate in society at large, it is a testament to the good intentions of Craigslist users, the effectiveness of our staff efforts, and perhaps realization among criminals that using Craigslist means getting caught, that so exceedingly few crimes have any connection with the 50 million Americans that use our community services each month," he said.

Last month, an Illinois sheriff sued Craigslist for allowing erotic services ads. Cook County Sheriff Tom Dart is one of many law enforcement authorities who have said that Craigslist may be luring women who would not have prostituted themselves if online services didn't make it so anonymous and convenient. However, similar ads have existed in newspapers for decades.

Craigslist posts safety tips that urge users to meet in public places, tell someone where they are, carry a cell phone, bring a friend, and trust their instincts.

Buckmaster has said in the past that the site's lawyers contend that federal laws protect Craigslist from being held liable for the actions of its users.

Specifically, courts have shielded Web sites from liability for user content through safe-harbor provisions in the Communications Decency Act. Those decisions are being challenged by prosecutors throughout the country, and protection is contingent on the Web sites not contributing to the problems. Craigslist and other Web sites cooperate with criminal investigations. In November, Craigslist announced a plan reached under an agreement with 40 attorneys general to combat prostitution ads on the site.

Now, the site is considering if there's anything more that can be done to increase safety for its users.

"Sadly, just as in society as a whole, terrible tragedies are possible despite everyone's best efforts to keep people safe," Buckmaster said. "We are evaluating this incident to see if there are any additional things we could do to better protect Craigslist users."