The Justice Department has seized the popular classified ads website over the sex trafficking ads on its website. On Friday, some visitors to the site were greeted with a message that said, " and affiliated websites have been seized as part of an enforcement action" by the FBI, the Postal Inspection Service and the IRS Criminal Investigation Division.

U.S. officials allege Backpage has earned more than $500 million in revenue from prostitution since it was founded in 2004.

The Justice Department says almost every sex trafficking case involves online ads, mostly from the website. Further, the department contends that the biggest issue concerning these websites is that they facilitate sex trafficking for people who would have been too sheepish to pursue sex on the streets, especially those looking for children. DOJ views this action as a major development in its overall effort to combat sex trafficking, although there have been previous cases against Backpage that were thrown out.

Another agency participating in the action is the Justice Department's Child Exploitation and Obscenity Section.

Last year, a Senate report alleged Backpage "knowingly concealed evidence" of child sex trafficking through its editing "by deleting words, phrases, and images indicative of criminality". Senate investigators said words like "young," "little girl" and "innocent" were removed while "the remainder of the ad would be published."

Backpage executives called to testify before Congress after the report was published took the fifth. The company accused senators of conducting a "witch hunt" against the site and said it cooperates with police departments to locate victims.

Some victims believe the federal action will help their civil lawsuits against Backpage. But others are concerned about the website shutting down because it could drive more child sex trafficking to the dark web and make it harder to find victims.

Reporting by CBS News' Paula Reid

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