Police believe the Birmingham, Alabama, gunmen used “conversion devices.” This means the following

CNN
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The people who opened fire in a mass shooting in Birmingham, Alabama, late Saturday are suspected of using “conversion devices” to carry out the attack. A preliminary police investigation found more than 100 shell casings were collected at the scene.

The shooting, which left four people dead and 17 injured, is just the latest outbreak of massive violence made possible by the small part known as a “conversion kit,” “Glock switch” or “auto-sear.”

Whatever you call these devices, they can transform a handgun into a fully automatic firearm. And they have proliferated rapidly in the United States in recent years, increasing the number of victims and raising new concerns for the public and law enforcement.

“Glock switches are the biggest public safety problem in our city and our state,” Birmingham Mayor Randall Woodfin said in a Facebook post on Sunday.

Birmingham Police Chief Scott Thurmond called the devices a “big problem,” adding that it was still being investigated whether the shooters used a switch or another type of weapon.

In Saturday's shooting, several people opened fire in Birmingham's busy entertainment district, in what police called a targeted “attack,” and several innocent bystanders were caught in the crossfire. No one has been charged, and authorities are offering a reward of up to $100,000 for information on the suspects.

“Fully automatic weapons, as the mayor said, do not belong on the streets of Birmingham or on the streets of the United States of America,” Thurmond said. “That's why it's very problematic today. They cause a lot of bloodshed very, very quickly. It's just a big problem.”

It's a small, inexpensive device that turns a semi-automatic pistol into an illegal machine gun, according to the U.S. Attorney's Office for the Northern District of Alabama. The device, made of either metal or plastic, attaches to the back of a firearm and allows the user to switch between semi-automatic and fully automatic fire without having to pull the trigger each time.

These switches allow shooters to fire a large number of shots in a very short space of time. The rapid fire and recoil also make aiming difficult, meaning that such shots can throw innocent bystanders into the spray, as apparently happened in Birmingham.

The devices are technically classified as machine guns under federal law and are illegal, but they can be manufactured using a conventional 3D printer, making it difficult for authorities to stop their production and distribution.

In recent years, switches have become much more common in mass shootings. According to a 2022 CNN analysis using data from ShotSpotter, a company that places audio sensors in U.S. cities to listen for gunfire, the number of instances of machine gun fire has risen from 400 in 2019 to 5,600 in 2021.

Automatic weapons have been used in several high-profile attacks in recent years, including a mass shooting in Sacramento, California, that killed six people and injured a dozen; a school shooting in Washington, DC, in which an automatic rifle-wielding sniper fired a barrage of more than 200 shots, wounding four people; and the killing of Houston police officer William “Bill” Jeffrey while executing a warrant in 2021.

There are various efforts underway in Alabama and across the country to stop the proliferation and use of these devices.

In July, the U.S. Attorney's Office for the Northern District of Alabama and the ATF announced “Operation Flip the Switch,” an initiative to target the conversion devices and prosecute their owners. As part of the initiative, the U.S. Attorney's Office partnered with Crime Stoppers of Metro Alabama to offer cash rewards for information leading to a warrant or arrest of anyone who owns or 3D prints switches.

“Machine gun conversion devices have no place in our community,” said U.S. Attorney Prim Escalona. “A switch may be small but it is deadly, and with a single pull of the trigger, many innocent people can be injured.”

Last month, five people in Tuscaloosa and the surrounding area were convicted of possession of percussion caps and other firearms offenses, the U.S. Attorney's Office said.

Woodfin, the mayor of Birmingham, pointed out that conversion devices are illegal under federal law but not under state law and called on the state legislature to ban the devices.

“Every mayor, police chief, sheriff and district attorney I know wants Glock switches banned,” he said. “Converting a semi-automatic weapon into a fully automatic weapon that fires all bullets within seconds does not belong on our streets.”

Democratic Rep. Phillip Ensler of Alabama, who represents Montgomery, has sponsored a bill to ban the devices at the state level. Last week, he attended a demonstration at a Montgomery shooting range where trained firearms experts tried (and failed) to aim the conversion devices.

“The demonstration showed that in many cases the bullets miss their target,” Ensler said in a phone conversation with CNN on Monday. “That's also why these weapons are so dangerous. The ammunition is discharged within a few seconds. It happens in such rapid fire, in such a rapid manner. It's very difficult to control and very dangerous.”

He has invited his colleagues to attend these shooting demonstrations so they can see the danger for themselves. And he said he sees bipartisan interest in the bill and hopes to pass it in the upcoming legislative session.

“I know this won't prevent every one of these tragedies, but it can prevent some of them,” he said. “I'm not naive, I don't think this will magically prevent every one of these horrific situations, but it can at least make things a little safer.”

There are also efforts to hold gun manufacturers accountable. This year, the city of Chicago filed a lawsuit against Glock, claiming that the gun manufacturer failed to prevent the use of conversion devices on its weapons.

“Glock designed its pistol so that a simple modification would convert it into a machine gun, and Glock knows that its pistols are frequently modified in this manner,” the lawsuit states. “Glock was warned by the (ATF) about the design of the weapon and knows that the problem could be fixed, but chose not to do so, thereby putting profit ahead of public safety and violating the law.”

Chicago police seized more than 1,300 modified Glocks between early 2021 and May 2024, the lawsuit says.

According to court documents, Glock has not yet responded to the lawsuit. CNN has asked Glock for a response.