More than a dozen Democratic senators demanded Thursday that Facebook CEO Mark Zuckerberg explain why his website's users appear to have little trouble engaging in private, unregulated gun sales more than four years after the company banned them.

In a letter sent Thursday morning, 13 senators led by Bob Menendez of New Jersey and Ed Markey of Massachusetts said they had serious concerns that Facebook's effort to eliminate private gun sales on its platforms had fallen short. Sens. Chris Murphy, Richard Blumenthal, Kamala Harris, Elizabeth Warren, Jack Reed, Mazie Hirono, Sheldon Whitehouse, Patty Murray, Dick Durbin, Amy Klobuchar and Cory Booker joined the letter, responding to an investigation by Protocol that found that gun owners are using thinly veiled, coded language to evade Facebook's algorithms and arrange gun sales in private groups.

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"We write to express concern that despite Facebook's ban of gun sales on its platforms, users are nonetheless able to facilitate firearm transactions in private Facebook groups specifically designed to skirt Facebook's ban on firearm sales," the letter reads. "In light of troubling reporting, we request information on the steps Facebook is taking to combat gun sales facilitated through private Facebook groups."

The senators acknowledged that Facebook has taken steps to proactively catch suspected gun sales, an effort the company undertook in 2016 after Congress failed to enact universal background checks in response to intense political pressure following the mass shooting at Sandy Hook Elementary School in Connecticut. However, the senators said Facebook's enforcement of its ban, based largely on a keyword-based algorithm and bolstered by users manually flagging posts for human review, isn't enough. They asked Facebook to respond to their questions, including how the company expects users to report violations within private groups, by March 12.

"Unfortunately, it is not enough to simply ban such sales. Effective monitoring, including the suspension of accounts in violation of these policies, is essential," the senators wrote.

Related: Facebook tried to ban gun sales 4 years ago. It's still trying.

A Facebook spokeswoman confirmed the company received the letter, but declined to comment. In the past, the company has said it remains committed to eliminating gun sales on its platforms, and has tweaked its algorithm to be more effective than when Facebook first banned the sale of guns and ammunition in 2016.

In most states, gun owners are legally allowed to sell firearms to each other without government regulation, sometimes referred to as the "gun show loophole." A handful of states, including California where Facebook is headquartered, require gun owners to complete sales through a licensed gun dealer, which holds onto the gun while it conducts a background check of the buyer.

Protocol's reporting earlier this month focused on obvious gun sales in the Virginia Gun Enthusiast group on Facebook, and there has been no indication that the company has tried to step in and restrict those exchanges. It appears to be business as usual in the group, ostensibly a "discussion group" for gun owners to talk about guns, similar to countless others across the country. In reality, the group serves as a digital swap meet where buyers and sellers meet and arrange sales via private message.

The Virginia Gun Enthusiast group on Facebook is still very active.Screenshot: Protocol

On Feb. 18, a new group member posted a message, "Interested in discussing the merits of the G19. Anyone want to debate?" in an apparent reference to the popular Glock 19 handgun.

"Pm me," a fellow group member responded, urging him to send a private message.

A Feb. 19 post demonstrated how easy it is for users to get around Facebook's keyword algorithm. "I'm thinking about getting a sub-2bills revolver; probably a point-3-eight. Anyone want to PM me some encouragement?" a user posted, spelling out numbers to ask about a .38-caliber revolver for less than $200.

A fellow group member quickly replied, clearing up any ambiguity by offering up a suggestion on where to look for such a gun. "You should take that dollar sign out of your post but armscor makes a rock island .38 for 210 beans which isn't bad."

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On Feb. 24, a user posted pictures of a semiautomatic AR-15-style rifle made by Heckler & Koch of Germany. The gun is shown with a carrying case and a half-dozen magazines filled with ammunition, under the words "Not for sale not for trade."

The first person to respond to the post wrote, "PM sent.. cool pics. Let's discuss."