When Marissa decided to switch jobs from nursing to prostitution a year ago, the online classified site Craigslist was pivotal, she said. Friends told her she could make lots of money just by advertising there.

I would never walk the streets. I would never do that kind of thing. But it's pretty easy to put an ad up. We love Craigslist, said Marissa, who declined to be identified by her full name because of her illegal work.

After this month's murder of a woman who advertised exotic services on Craigslist in Boston, Marissa now carries a weapon, the latest illustration of how Craigslist has evolved from humble e-mail to a few San Francisco friends in 1995 to a global phenomenon associated with illicit sex and murder.

Legal experts say growing scrutiny of Craigslist by authorities could lead to big changes at the 14-year-old online bazaar that generates more than 20 billion page views per month in 50 countries with a staff of just 28 people.

The April 14 murder of 26-year-old masseuse Julissa Brisman, who was bashed in the head and shot three times, is the latest headline-grabbing crime linked to Craigslist.

Philip Markoff, a 23-year-old Boston University medical student, was charged with killing Brisman and with an April 10 attack on a 29-year-old woman who advertised sex services on Craigslist in the Boston area. Dubbed the Craigslist killer, Markoff is being held in an isolated cell on suicide watch.

The murder followed the killing of George Weber, a New York reporter who was knifed to death after responding to a personal ad he placed on Craigslist in March, and the early-April sentencing of Michael Anderson, a Minnesota man convicted of killing a woman who responded to a babysitting ad. He also became known as the Craigslist killer.

There's a whole field of law emerging which is online media liability law and the question is how much liability do we place on companies that host information other people post online, said John Palfrey, a Harvard Law School professor.

The general policy approach we have taken to the Internet starting a decade plus ago was to say there is basically no liability, but these recent cases put to the test that policy, said Palfrey, who is a co-director of Harvard's Berkman Center for Internet and Society.

Is it appropriate to say, 'No matter how harmful the material is, we don't want to have any obligation placed on Craigslist to take it down'? My sense that we need to strike a balance here between supporting online businesses and supporting innovation while also achieving greater levels of safety for children and others, he said.

That's the conversation that these awful events provoke, he added, noting such changes would have a huge impact on Craigslist and on future businesses that are structured in a similar way. It would be very hard to run if it was as lean a staff as Craigslist does and with the extraordinary reach that it has. That is exactly the trade off.

'HORRIFIC, BRUTAL TRAGEDY'

Along with its free listings for just about anything -- from apartments to furniture, jobs and cars -- San Francisco-based Craigslist.org provides one of the largest and most controversial sex-service listings.

It is partially owned by online auctioneer eBay, which bought a 25 percent stake in 2004.

Connecticut Attorney General Richard Blumenthal, who led a probe into American International Group's controversial $165 million in bonuses, this week called on Craigslist to block ads for escort services that promote prostitution.

He sent a letter to Craigslist officials asking them to eliminate photographs in the erotic services and similar sections of the site, hire staff to screen ads that blatantly violate Craigslist rules and offer incentives for people who flag and report prostitution advertisements.

Jim Buckmaster, Craigslist chief executive, initially denied his site offered sex-related ads, but he changed tact in a blog posted on Wednesday where he wrote that more must be done by Craigslist to eliminate illegal activity on its site.

Craigslist is fully engaged in pursing this goal, and has several initiatives underway that speak to the concerns addressed in Mr. Blumenthal's letter, concerns which we also share, Buckmaster said.

Under pressure from 40 U.S. attorneys general, Craigslist agreed in November to charge people posting erotic ads $5-$10 by credit card and require them to submit a working phone number to use the site. Blumenthal said that allowed authorities to track illegal activity.

But Azer Bestavros, a Boston University computer science professor, doubts such sites can ever be policed fully.

One can argue that Craigslist is just facilitating communication, he said. Are we going to punish the phone company because they allowed prostitution by allowing people to call each other?

Marissa, who describes herself in her ad as very hot, exciting, engaging and full of sensuality, said she has her own ways of screening clients: she won't meet men unless she can call them at their workplace first.

If they don't allow me to do that, I don't see them. But 90 percent of them let me call them at work, she said in a telephone interview as she drove between customers. Most of us know how to take care of ourselves.

(Editing by Anthony Boadle)

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