Federal officials policing American ports are recovering record numbers of illegal conversion kits that enable some semiautomatic handguns, including several popular models made by Glock, to shoot fully automatic.

The U.S. Customs and Border Protection first encountered the devices in 2016. That year, agents seized 22 full-auto pistol conversion kits at all 328 American ports of entry, according to CBP data provided to The Trace. A year later, the number of recoveries dropped to seven. But in 2018, border agents seized 301 kits, a more than six-fold increase over 2016. The majority of the devices seized by the CBP have arrived in air parcels from Asia, according to a spokesperson.

“These products are heavily regulated, and there are serious consequences for not intercepting them,” said Jesus Sanchez, the CBP’s assistant port director at Los Angeles International Airport.

Efforts by law enforcement to track down conversion kits were first reported by reported by CNN in May.

The federal government classifies devices or parts that turn semiautomatic firearms into fully automatic weapons like machine guns. Civilian possession of machine guns has been strictly regulated since the passage of the National Firearms Act of 1934, which established that fully automatic firearms and conversion devices must be registered with the Bureau of Alcohol, Tobacco, Firearms and Explosives.

“The conversion is relatively fast and simple, resulting in a rate of fire of approximately 1,200 rounds per minute,” said Scott Curley, an ATF spokesperson. In contrast, semiautomatic weapons can only fire as fast as the user can pull a trigger, typically around 60 rounds per minute.

Sanchez compared the conversion kits to bump stocks, the rapid-fire devices that were outlawed after their use by the perpetrator of the 2017 Las Vegas mass shooting that left 58 dead.

Despite law enforcement’s best efforts to intercept fully automatic conversion kits, it’s likely that thousands are making it through American ports undetected. ATF agents have identified 3,800 PayPal transactions between U.S. residents and a single Chinese company selling the kits, according to a May federal court affidavit. Officials further identified 2,900 packages containing the conversion devices that had been shipped to the United States under false customs declarations.

“We X-ray every parcel that comes into the country. We’re trained to intercept this kind of item,” Sanchez said. “But even if you X-ray the package, it could just look like hardware.”

The supply of machine guns in the United States has dwindled since the 1986 passage of a federal law that banned the manufacture of new fully automatic weapons for civilian sale. And their use in crime is rare.

But customs officials say e-commerce and trade have undermined existing regulations. A number of little-known companies and e-commerce platforms or apps like Wish, a Chinese shopping app, sell the kits to American customers with no notice or warning that the products are illegal in the United States. “It’s so easy to buy these products that people don’t realize they’re doing anything illegal,” Sanchez said.

For a time in 2017, a handful of companies based in Asia even sold full-auto conversion kits on mainstream platforms like Amazon (the website subsequently removed the listings). There are also some American companies that sell partially completed conversion kits which are technically legal as sold, but become illegal if finished.

Sanchez said that investigators for the Department of Homeland Security — the CBP’s parent agency — have asked governments in countries like China to crack down on businesses shipping conversion kits to American customers. But ultimately, he said, law enforcement has no authority to cut off the supply of the devices at their source, since “the product may be legal elsewhere.”