"Either they find a way to police this... or we're going to need to take that section of the Web site down," Madigan said.
The threat from McMaster is the latest move in an ongoing effort to combat allegedly illegal activity online and comes
"Recent national events, along with ongoing law enforcement efforts in South Carolina, indicate that craigslist has not installed sufficient safeguards since November to prohibit the Internet site from being used as a vehicle to advertise or solicit prostitution," McMaster wrote.
Philip Markoff, 23, is suspected of using Craigslist to lure three women to upscale hotels, where he allegedly robbed them and killed one of them. Investigators believe Markoff contacted the women through Craigslist ads in which they offered erotic massages. Markoff, who has been dubbed the "Craigslist Killer" by the media, has pleaded not guilty.
Earlier this month, Michael John Anderson, 20, of Savage, Minn., was sentenced to life in prison for killing Katherine Ann Olson, who had responded to an ad for a nanny that Anderson placed on the site. And New York radio reporter George Weber was stabbed 50 times in his Brooklyn apartment earlier this year, allegedly murdered by a man who answered Weber's ad on Craigslist, offering $60 for "rough sex."
In a statement released last week, Craigslist CEO Jim Buckmaster said, "We look forward to meeting with Attorney General Madigan to discuss any and all ideas she and others may have for further eliminating illegal activity from Craigslist, while preserving all of the functionality and positive attributes that are valued by the overwhelmingly law-abiding Craigslist community of users."
In an interview last month with ABC News, Craigslist founder Craig Newmark denied that the site "facilitates" prostitution and said he has no plans to change the "erotic services" section.
Buckmaster told "Nightline" that the Craigslist community -- about 50 million people use the site a month -- has a low incidence of crime. "Now, the risk is not zero, and no occurrence of violent crime is acceptable," Buckmaster said.
"I'm very proud that our site is composed of people who are overwhelmingly trustworthy and good. I am very proud that there is very little crime on our site, proportionately," Newmark said. "Compare that to any other American community. Look at the numbers."