CHICAGO (AP) - The Cook County Sheriff's Department says it's suing Craigslist, calling the popular Web site the largest source of prostitution in the U.S.
Sheriff Tom Dart is scheduled to detail the lawsuit at a news conference Thursday at noon.
A call for comment before business hours to Craigslist's public relations firm was not immediately returned.
Craigslist has come under fire in recent months for the "erotic services" ads that are posted on its Web site.
There have been several arrests of people on prostitution charges after posting the ads online.
In November, Craigslist pledged to crack down on prostitution ads as part of an agreement with several attorneys general. It said it would require anyone who posts such an ad to provide a working phone number and pay a fee with a valid credit card.