Craigslist may be best known for online classified ads that 40 million Americans use every month to find apartments, jobs and used furniture -- but today Sheriff Tom Dart of Cook County, Ill., demanded that Craigslist "stop being the largest source of prostitution in America."
A glance at the online "erotic services" ads leave little to the imagination.
Recent banner ads read "Teens for Cash" and "Hot young girls needed for Naughty Nanny Service."
"It's abundantly clear it's prostitution," said Dart. "You have pictures that are pornographic. You have people setting out their rates of what they charge for what services and what amounts of time."
Dart wants to ban the erotic services ads. He filed a federal lawsuit against Craigslist today, claiming that the erotic ads on the classified site are a public nuisance and damage the property, health and safety of the community.
Dart said that the majority of the prostitutes jailed by his department -- and other law enforcement agencies nationwide -- do business on Craigslist. A recent bust in Florida netted 35 arrests.
"The marketplace for prostitution has shifted. It's going away from what we commonly see on the street to online," said Salim Goswani of the Chicago Alliance Against Sexual Exploitation. "And Craigslist is the biggest facilitator of this."
In an e-mail to ABC News, Craigslist said that it is taking measures to prevent illegal activity on the site, including the removal of inappropriate ads.
"Misuse of Craigslist to facilitate criminal activity is unacceptable, and we continue to work diligently to prevent it," the company said. "Misuse of the site is exceptionally rare compared to how much the site is used for legal purposes. Regardless, any misuse of the site is not tolerated on Craigslist."
With only a 28-person staff and more than 30 million new classified ads posted every month, it's nearly impossible for Craigslist to monitor and review each individual ad.
"It's a very big burden to place on them to be the ones to police the content that's on their site," said Evan Brown, an Internet lawyer at Hinshaw & Culbertson.
Instead, the company said its site is "community-moderated," and it relies on users to flag and remove problematic postings.
But in an interview for ABC's Nightline in 2006, Craigslist founder Craig Newmark defended keeping racier sections of the site, like "erotic services," intact.
"People would find other ways to do it," he said. "It would spread to other, more legitimate sections of our site."
Last November, in a settlement with attorneys general from 40 states, Craigslist agreed to crack down on "erotic services" ads, requiring anyone who places an ad to give a working contact number and pay via credit card. If subpoenaed, the site would hand that information over to law enforcement agencies.
"On a daily basis, we are being of direct assistance to police departments and federal authorities nationwide," Craigslist said in a statement. "Craigslist is an extremely unwise choice for those intent on committing crimes, since criminals inevitably leave an electronic trail to themselves that law enforcement officers will follow."
Despite efforts to get rid of illegal ads, a 19-year-old girl, who asked to keep her identity private, was arrested for prostitution last month when an undercover officer responded to an ad posted by her pimp on Craigslist.
"It's ruined a lot of other people's lives," she said. "And I know what it's done to me, it's done to a lot of other ladies."