(CNN) -- An Illinois sheriff filed a federal lawsuit Thursday against the owners of craigslist, accusing the popular national classified-ad Web site of knowingly promoting prostitution.

"Craigslist is the single largest source of prostitution in the nation," says Cook County Sheriff Thomas Dart.

Cook County Sheriff Thomas Dart wants craigslist to eliminate its Erotic Services section.

"Craigslist is the single largest source of prostitution in the nation," Dart said. "Missing children, runaways, abused women and women trafficked in from foreign countries are routinely forced to have sex with strangers because they're being pimped on craigslist."

The FBI found more than 2,800 child prostitution ads posted on craigslist, with Chicago, Illinois, in the top 10 cities for juvenile prostitution, Dart said.

The sheriff also noted instances of what he said was child neglect while parents were engaged in activity solicited on the Web site.

In an e-mailed statement, craigslist told CNN it had not seen Dart's complaint but that "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."

"On a daily basis, we are being of direct assistance to police departments and federal authorities nationwide," spokeswoman Susan McTavish said.

"Misuse of craigslist to facilitate criminal activity is unacceptable, and we continue to work diligently to prevent it," she 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."

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But Dart said that the Web site for two years has ignored his written requests to shut down the Erotic Services section and failed to take action or adequately monitor the section despite hundreds of arrests.

"Pimps are preying on the most vulnerable members of our society and taking advantage of our struggling economy," he said. "The worst part is craigslist's owners know their Web site is still being used for illegal purposes and they're doing nothing to stop it."

Craigslist entered into an agreement with 43 states' attorneys general in November to enact measures that impose restrictions on its Erotic Services section. The agreement called for the Web site to implement a phone verification system for listings that required ad posters to provide a real telephone number that would be called before the ad went public.

Craigslist also imposed listing fees, requiring a credit card, for ads in the section. The proceeds were to be donated to charity.

Dart called the fees "dirty money" and said the move was a "publicity stunt" that had little practical effect because pimps use stolen credit cards or post ads in free sections.

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