Facebook told a group of U.S. senators that its years-long battle to eliminate peer-to-peer gun sales across its platforms "is not done" and that it will "continue to strengthen" enforcement. The 13 lawmakers had publicly sent the questions to Facebook following a Protocol investigation in February, which found that private groups on the website remain a popular place to buy and sell firearms despite Facebook's 2016 ban on gun sales.

Facebook's response, however, leaves many key questions unanswered, including how many gun groups it has removed from the site for violating its policies and how it expects users to report suspected gun sales in closed groups. In the letter, signed by VP of public policy Kevin Martin, Facebook said it did not track how many private groups are suspended for violating its firearms policy, which prohibits the sale or transfer of guns, gun parts, ammunition and 3D-printing files for firearms.

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Martin said Faceook had "policies in place to protect against repeat offenders and recidivist behavior," but he did not provide details. "While we cannot provide specific details lest bad actors use the information to game the system," he wrote, "Facebook would not allow group administrators to set up a new group similar to a previously removed group."

The back and forth between Facebook and Congress represents the latest in a line of content moderation challenges for the company, underscoring how difficult it is to maintain user privacy and engagement while also banning certain behavior.

It's unclear whether members of Congress will seek further answers from Facebook on how it is working to eliminate gun sales, which have persisted despite numerous attempts to prevent users from using the website to arrange private, unregulated transactions. When Facebook announced its ban on gun sales in January 2016, it effectively took a political stand against the so-called "gun show loophole" that Congress could close by enacting universal background checks.

Democratic Sen. Bob Menendez, who along with Sen. Ed Markey led the latest letter from concerned senators, voiced his displeasure with Facebook's response. "Facebook's non-answer answer is disappointing, but not surprising. I really hope Facebook can do a better job for the sake of the American people, and use their power to keep users safe, connected and well-informed. That's what we need and deserve, especially in challenging times like the ones we're living with the coronavirus pandemic," Menendez told Protocol.

Protocol sent more than a dozen detailed questions to Facebook seeking to clarify the company's responses. A Facebook spokesperson declined to address any of the questions, saying, "We have no additional comment beyond what is in the letter."

Facebook's approach is similar to how the company has handled inquiries from congresspeople in the past. The latest letter bears striking resemblance to one Facebook sent Sen. Markey in 2016 following criticism in the press over the company's ability to enforce its ban on gun sales. Since then, Facebook has tweaked its AI-based algorithm to automatically detect suspected gun-sale posts numerous times, in part to better detect thinly veiled coded language that's become popular among users.

Common examples include uploading a photo of a gun with the words "Not for sale/Not for trade, just want to discuss" or "PM me to discuss." The approach is meant to shift the conversation to private messaging where, as Protocol demonstrated, users are often eager to discuss terms of a sale or trade.

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In its letter to Sen. Menendez, Facebook addressed changes in its algorithm generally, saying that its "proactive AI technology is able to detect content that includes images of firearms and signals that indicate an intent to sell (including posting contact information such as phone numbers and usernames for other social media accounts and price information)."

As Facebook continues to try to enforce its ban, at least one group has been removed in recent weeks for violating it. The Virginia Gun Enthusiasts group, which Protocol highlighted as an example of how easily users are able to continue to skirt Facebook's ban, has been shut down. Users who try and access the page instead get a blank screen.

A Facebook spokesperson confirmed the group was removed for violating its policies.