A group of current and former Bureau of Alcohol, Tobacco, Firearms and Explosives employees has written to Congress proposing a way to change current law that would make bump stocks illegal, and pushing back against critics who said it was the ATF’s job to ban the devices in the first place.

Michael Bouchard, president of the ATF Association, said he sent the letter to several key lawmakers on Friday, including Charles Grassley, a Republican from Iowa who chairs the Senate Judiciary Committee, and the legislators who introduced a bill to ban bump stocks in the wake of the mass shooting that killed 58 people and wounded hundreds more in Las Vegas this month.

“Recently, some have attempted to cast blame on ATF for not banning devices like the ‘bump slide’ used in the Las Vegas shootings,” the letter says. “We would like to clarify this confusing issue to protect honorable ATF employees from false allegations that they chose to make this item legal when it was the law that prohibited them from regulating the item.”

On Thursday, Rick Vasquez, who was the ATF’s chief technical expert when bump stocks were considered in 2010, told The Trace that his group’s decision had been sound, and that they did not have the authority to regulate the devices.

Law enforcement officials have said they found at least a dozen bump stocks attached to rifles in the Las Vegas shooter’s hotel room. The devices would have allowed him to fire more rapidly, wounding and killing more people in a short period of time.

Last week, as legislators began discussing changing the law to ban bump stocks, Wayne LaPierre, the National Rifle Association’s executive vice president, said the responsibility of regulating bump stocks fell squarely on the shoulders of the ATF. The White House counselor Kellyanne Conway also blamed “Obama’s ATF” for failing to make bump stocks illegal.

Speaker of the House Paul Ryan has sided with the NRA, calling this week for a “regulatory fix,” not new legislation.

Under federal law, machine guns are strictly regulated, and have been out of production in the United States since 1986.

Bouchard said in the letter that the law is very clear: It defines a machine gun as any weapon that is capable of firing multiple rounds with the single pull of the trigger. A bump stock still technically requires the shooter to engage the trigger every time the gun is fired, which is why the ATF had previously determined that the devices were not subject to regulation.

Bouchard said that the ATF had made the correct decision, and that the agency did not have the authority to overturn the previous ruling.

“We … hope you will not allow the honorable employees of ATF, who followed existing law in their bump stock ruling, to be falsely accused of not doing their job by those who seek to exploit the situation for political gain,” Bouchard said in the letter.

Bouchard said that one fix to the law, if legislators want to ban bump stocks, would be adding a new category to the National Firearms Act of 1934 allowing for the regulation of “multi-burst trigger activators,” which are already regulated in New York and California.

“The new category of Federal law would encompass other accessories on the market that make semi-automatic rifles fire like a machine gun but are engineered in a way to avoid regulation under current Federal law,” the letter said.

Bouchard said in an interview on Saturday morning that members of the ATF Association, a 400-member group comprised of current and former agency employees, began discussing what action to take about a week ago.

“Our language won’t allow people to keep trying to skirt the law,” he said.

Two pieces of legislation have been introduced so far with the aim of banning bump stocks. The first, by Sen. Dianne Feinstein, a Democrat from California, would ban the sale, transfer and manufacture of bump stocks, trigger cranks and other accessories that can accelerate a semiautomatic rifle’s rate of fire.

The second proposed ban, introduced by Rep. Carlos Curbelo, a Republican from Florida, would apply just to bump stocks.