SAN FRANCISCO (KCBS)— San Francisco has joined five other major U.S. cities in supporting a federal lawsuit against a prominent national website known for posting “escort” service advertisements.The city attorney’s office says the site actually promotes prostitution and the commercial sexual exploitation of children.

A federal lawsuit was filed in Massachusetts against in October on behalf of three teenage girls who alleged the site facilitated meetings that led them to be raped thousands of times collectively over a period of years.

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San Francisco Deputy City Attorney Elizabeth Peterson said the city has now joined Atlanta, Denver, Houston, Philadelphia and Portland in filing a friend of the court brief in support of the suit.

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“Essentially they charge people to advertise what they allege are escort services, but in reality these are prostitution ads and approximately 10 percent of them are for underage children,” Peterson said.

She added that is similar to Craigslist, which voluntarily removed its escort section in 2012. The said the cities supporting the suit are among the countries top markets for the sex trafficking of minors.

“Of course this is a devastating problem for all cities to realize that their most vulnerable citizens are being victimized in this manner,” she said.

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Backpage has moved to dismiss the suit based on federal law that protects neutral Internet service providers.