When the first bump stock came across Rick Vasquez’s desk in 2010, he knew that his evaluation would take longer than usual.

As the senior technical expert for the Bureau of Alcohol, Tobacco, Firearms and Explosives, it was Vasquez’s job to help give opinions about whether new guns and gun products would be legal under federal law.

The bump stock, manufactured by a company called Slide Fire Solutions, did something novel: It used the recoil from firing a round from a semiautomatic rifle — as opposed to a spring or a crank — to make it fire at nearly the same rate as a machine gun.

The ATF defines a machine gun as any weapon that is capable of firing multiple rounds with the single pull of the trigger. Under federal law, machine guns are strictly regulated, and have been out of production in the United States since 1986.

The question before Vazquez and his team: Did a bump stock transform a rifle into a machine gun, making it illegal?

After months of testing the devices and studying the law, the ATF arrived at a decision. It ruled that the bump stock did not make a gun fully automatic, because the trigger of a rifle equipped with the device still had to be engaged every time the weapon fired.

“We could not find a way to classify it as a machine gun,” Vasquez said. He explained the decision in detail in a document he shared with The Trace.

Slide Fire received a letter from the agency on June 7, 2010. A copy of the document remains on the company’s website.

Vasquez thought he’d long since left the decision behind him. Then, last Sunday, a dozen bump stocks were recovered in the Las Vegas hotel room from which a gunman carried out the worst mass shooting in modern American history.

In the wake of the massacre, which left 58 dead and hundreds injured, Democratic Senator Dianne Feinstein introduced a bill that would ban bump stocks and other devices meant to simulate automatic fire. Soon after, Wayne LaPierre, the National Rifle Association’s executive vice president, said bump stocks should be subject to “additional regulations.” But unlike members of Congress, including some Republicans, LaPierre said the responsibility of regulating bump stocks fell squarely on the shoulders of the ATF.

“Despite the fact that the Obama administration approved the sale of bump fire stocks on at least two occasions, the National Rifle Association is calling on the [ATF] to immediately review whether these devices comply with federal law,” read a joint statement from LaPierre and Chris Cox, the gun group’s top lobbyist.

The ATF “needs to do its job,” LaPierre said later on Face the Nation.    

The White House counselor Kellyanne Conway also blamed “Obama’s ATF” for failing to make bump stocks illegal in a CNN interview, saying the agency “decided not to regulate the device.”

Vasquez says he found those comments galling.

“We did the right thing by the letter of the statutes,” he said. “There’s a tragedy that happened and nothing can change that. But to try to put the blame on us, it really irritates me.”

Vasquez, who was assistant chief of the ATF’s Firearms Technology Branch at the time, said the idea that the Obama administration was involved in the approval process doesn’t make any sense. For starters, he said, Obama advocated for more gun regulation, not less.

He insisted his team did what it always did: tested the product; consulted applicable laws, including the Gun Control Act of 1968 and the National Firearms Act; and wrote an evaluation. “It was a monumental job,” he said.

Vasquez said that agency higher-ups also weighed in, though he does not recall exactly who. In any case, he said, no one up the chain of command disagreed with his team’s initial ruling. The bump stock could not be classified as falling subject to federal law banning the sale of new machine guns.

Asked now whether federal law should be changed to explicitly ban the devices, Vasquez said, “it’s not my place to make that call.”