By Sarah N. Lynch and Lisa Lambert

WASHINGTON (Reuters) - U.S. law enforcement agencies have seized the sex marketplace website Backpage.com as part of an enforcement action by the Federal Bureau of Investigation, according to a posting on the Backpage website on Friday.

A Phoenix FBI official said that there was "law enforcement activity" on Friday at the Sedona, Arizona home of Michael Lacey, one of the founders of Backpage.

Glenn Milnor, FBI Special Agent and Media Coordinator, referred further inquiries to the Department of Justice.

The Department of Justice will provide more information at 6 p.m. EDT (1000 GMT), according to the website posting, which said U.S. attorneys in Arizona and California, as well as the Justice Department's section on child exploitation and obscenity and the California and Texas attorneys general had supported shutting down the website.

Lawmakers and enforcement officials have been working to crack down on the site, which is used primarily to sell sex and is the second largest classified ad service in the country after Craigslist.

Backpage and advocacy groups have argued that the ads are free speech protected by the U.S. Constitution.

The U.S. Senate passed legislation last month making it easier for state prosecutors and sex-trafficking victims to sue social media networks, advertisers and others that fail to keep sex trafficking and other exploitative materials off their platforms.

Launched in 2004, Backpage has affiliates across the country and around the world, and by 2014 brought in annual revenue of $135 million, the New York Times has reported.

The Supreme Court in January 2017 refused to consider reviving a lawsuit against Backpage.com filed by three young women alleging the site facilitated their forced prostitution. But the site has since then faced a slew of other lawsuits alleging child sex trafficking.

The National Center for Missing and Exploited Children has told Congress that nearly three quarters of the cases submitted to the center relate to ads posted on the site. The state of California has said that 90 percent of the site's income were attributable to "adult ads."

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At the end of 2016, Texas and California attorneys general raided the company's Dallas headquarters and arrested chief executive Carl Ferrer and other former company executives on pimping-related charges. But the judge in the case ruled that the site was protected by the First Amendment, and the site should not be liable for the speech of third parties.

The states' attorney generals did not immediately comment on Friday's seizure.

(Reporting by Sarah Lynch and Lisa Lambert, Additional reporting by David Schwartz in Phoenix, Editing by Toni Reinhold and Rosalba O'Brien)