A Cook County Sheriff in Chicago, Illinois sued online classified site Craigslist on Thursday, dubbing it the single largest source of prostitution in the nation, according to media reports.

After regularly busting alleged prostitution rings that operated on Craigslist, Sheriff Thomas Dart said he has had enough with the sites poor regulation system.

Dart held a press conference at noon today to discuss the lawsuit, which asks a federal judge to order the site to close down its Erotic Services section.

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, Dart said in a news release.

Dart cited ads such as Teens for cash ... $100 quickie and Are you looking to make some good money in this tough economy. Minimum pay is $25 per hour ... performing duties specified in Naughty Nannies handbook.

In recent weeks, Dart said, one of his own officers — writing as a 15-year-old girl — posted ads for sex on the site that generated responses from several people, including a convicted sex offender.

Last June, the Chicago department arrested 76 men and women on prostitution-related charges in a Craigslist sting. Undercover officers had posed as customers who scheduled “dates” with women that posted ads in the “erotic services” section on the website. The sting, called Operation Quiet Money, lasted for three months.

In December 2007, four Chicago-area men were charged with using the site to run a prostitution ring that involved girls as young as 15.

In another case in New York, Federal prosecutors accused a man in November on charges of being a violent pimp who forced young girls and women into prostitution. Prosecutors say he advertised the services of women between the ages of 15 and 20 on Craigslist and money off of them as they paid him commission.

Soon after that incident, Craigslist reached an agreement in November with attorneys general in Connecticut, Illinois and several other states that had called for the company to crack down on prostitution ads.

Jim Buckmaster, Craigslist's CEO, said it would continue to allow legitimate escort services to continue advertising, while discouraging illegal activity by requiring anyone posting erotic services ads to provide a working phone number and pay a fee with a valid credit card.

These measures, however, seem to have been ineffective as the problem still prevails today.

San Francisco-based Craigslist did not immediately respond to an e-mail message seeking comment on the matter.