(CBS NEWS) -- A bipartisan group of 47 lawmakers – almost half of the Senate – is co-sponsoring a bill to close what it sees as a loophole that supports an online market for sex trafficking. They want to remove legal protections for websites that host prostitution ads, but some in law enforcement warn the effort could be counterproductive.

The National Center for Missing and Exploited Children says classifieds website Backpage.com is involved in nearly three-quarters of all reports of online child sex trafficking that it receives from the general public, reports CBS News correspondent Anna Werner. A Senate investigation showed adult ads proved to be big business for Backpage, generating more than 90 percent of its ad revenue in 2011. Some victims' advocates want the site shut down. But is it that simple?

Chicago police use Backpage for undercover sting operations. They post ads on the site and according to Commander Bill Leen, who heads Cook County's vice unit, they get responses within a minute or two. Leen said sometimes they'll get 50 to 100 calls in one night from one of their ads. That's despite the fact that Backpage shut down its "adult" advertising section earlier this year under pressure from lawmakers. Now, police say they find ads for sex with young girls in the website's dating section.

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