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CHICAGO (AP) — A federal judge has thrown out a lawsuit aimed at driving classified ads for prostitution off the Craigslist Web site.

U.S. District Court Judge John Grady said not all postings on the Craigslist "Adult Services" section advertised prostitution and that other services could legitimately appear there.

"A woman advertising erotic dancing for male clients is offering an adult service, yet this is not prostitution," he said in a 31-page opinion Tuesday.

He said such an ad "might even be entitled to some limited protection under the First Amendment" to the U.S. Constitution.

Grady also said Craigslist is not to blame if those who post ads violate the San Francisco-based Web site's guideline barring prostitution ads.

Cook County Sheriff Tom Dart filed the lawsuit against Craigslist in July, saying a change in the category name from "Erotic Services" to "Adult Services" had failed to deter prostitutes from using the site.

"Sheriff Dart may continue to use Craigslist's Web site to identify and pursue individuals who post allegedly unlawful content," Grady said. "But he cannot sue Craigslist for their conduct."

Dart said in a telephone interview that the sheriff's department has made hundreds of prostitution arrests related to Craigslist in the last two years. He said his investigators scan the list for names and phone numbers, arrange to meet prostitutes at hotels and arrest them when they offer sex for money.

"It isn't that I just woke up one morning and said, 'Let's sue Craigslist,'" Dart said. "This came after two years and hundreds of arrests off of the Web site, and many of the arrests involve juveniles and human trafficking."

He said he was still deciding whether to appeal.

Responding to the ruling, Craigslist CEO Jim Buckmaster said through a spokeswoman: "We welcome Judge Grady's decision on this matter."

Dart and other critics acknowledge that when Craigslist pulled the plug on its "Erotic Services" category a number of pictures of scantily clad women in suggestive poses disappeared from the Web site.

But Dart says its plain that prostitution is still being advertised.