The Senate report, pushed by Sens. Claire McCaskill, D-Mo., and Rob Portman, R-Ohio, alleges that Backpage automatically filters out words in ads that could indicate that the site was offering sex with children, undermining the controversial website's First Amendment defenses.

That act and others changing the wording of prospective advertisements on the site undermine the company's free-speech defense that it is simply a host for other ad buyers' content, and therefore protected under federal law, the new report by Portman, R-Ohio, and McCaskill, D-Mo., alleges. It was released Monday evening.

Within four hours, Backpage had shut down its adult section and had put "censored" in large, red letters.

The site included links to an online freedom organization and a children's advocacy organization that is quoted as saying the pressure on Backpage will hurt the government's efforts to track down sex traffickers.