The sheriff of Cook County, Ill., on Thursday filed a lawsuit in federal court charging Craigslist.org with facilitating prostitution.

The sheriff, Thomas Dart, is seeking an injunction to prevent further erotic services ads from being posted on the site, the award of compensatory and punitive damages to be determined at trial, and reimbursement for the more than $100,000 in costs incurred policing prostitution-related crime over the past year.

In its defense, Craigslist might argue that its ads make tracking down prostitutes easier, thereby decreasing the cost of investigations.

The complaint filed against Craigslist says that while the site does not profit directly from prostitution, the popularity of its erotic services ads is the reason Craigslist is the ninth-most-popular Web site in the country and the reason the site can charge up to $75 for job ads.

"Advocacy groups confirm the popularity of Craigslist's erotic services," the complaint states. "The Polaris Project, a group against human sexual trafficking, believes Craigslist is now the single largest source of prostitution, including child exploitation, in the country." It also cites another group, Love 146, which reports that Craigslist is used for child prostitution.

The complaint claims that authorities across the country have reported Craigslist's role in facilitating sex trafficking. "In one such instance, the FBI uncovered a sex ring involving child prostitutes in which the pimps 'posted over 2,800 advertisements on Craigslist.org,' " the complaint states.

This is not the first time Craigslist has been confronted with such claims. Sheriff Dart says that he has sent five separate letters to Craigslist asking the site to close its erotic services section, to no avail. In March 2008, Connecticut Attorney General Richard Blumenthal sent a letter to Craigslist's attorney demanding that Craigslist police its users' postings.

"I am astonished and appalled by Craigslist's refusal to recognize the reality of prostitution on its Web site -- despite advertisements containing graphic photographs and hourly rates, and widespread public reports of prostitutes using the site," Blumenthal wrote.

That letter resulted in a joint statement issued by Craigslist and 40 state attorneys general last November in which those placing erotic services ads have to pay a $5 to $10 fee with a credit card, with that money going to combat child trafficking and exploitation.

According to the complaint, the agreement has had no perceptible effect, and the rate of prostitution arrests related to Craigslist has remained unchanged.