Nathaniel Berhow was too young to own a gun. But the 16-year-old was able to get one anyway: a .45-caliber semi-automatic pistol assembled from a DIY “ghost gun” kit — likely ordered online — which he used last month to shoot five classmates at his Southern California high school before turning the gun on himself.

Ghost guns are the Wild West of gun policy because they aren’t subject to federal background checks, and the components don’t have serial numbers, making them next to impossible to trace back to their owner when they’re recovered at crime scenes.

The ATF used to regulate “ghost guns,” because they held that receivers, a core firearm component that comes in the kits, were subject to the same rules as any other type of gun. But in 2006, as the online marketplace for the kits boomed, the ATF changed course. Now, 13 years later, ghost guns are showing up at crime scenes across the country and posing a major challenge to law enforcement.

Everytown for Gun Safety, the gun control advocacy group founded by 2020 candidate Michael Bloomberg, wants to know why ATF won’t crack down on ghost guns, since they had regulated them in the past. On Thursday, Everytown filed a petition with ATF asking it to “correct its own failure to regulate untraceable ghost guns.”

ATF did not respond to VICE News’ request for comment.

Ghost guns are typically sold in kits online, costing between $250 to $500, with parts that can be easily assembled into a fully-functioning firearm — meaning anyone can build their own high-powered AR-style rifle, and the government will never know about it.

Initially a niche hobby enjoyed by firearm enthusiasts, these kits have become attractive to criminals or criminal enterprises because of the untraceability. According to ATF, 30% of firearms recovered at crime scenes in California in an unspecified period are ghost guns.

“Whether it’s made from a kit with minimal assembly or purchased fully assembled, a gun causes the same devastation in the wrong hands,” Eric Tirschwell, managing director of litigation and national enforcement at Everytown Law, said in a statement. “With this petition, we’re not just saying that ATF is uniquely positioned to keep ghost guns out of the wrong hands.”

The ATF’s regulation or nonregulation of ghost guns hinges on whether a receiver is “finished” when it’s sold as part of a kit. A “finished” receiver means it already has a trigger cavity drilled into it; unfinished receivers require buyers to drill it in themselves. Ghost-gun kits containing unfinished receivers are marketed as an 80%-complete firearm, which the ATF describes as having “not yet reached a stage of manufacture that meets the definition of firearm frame or receiver found in the Gun Control Act of 1968.”

“It’s arbitrary and capricious to engage in this line-drawing exercise,” said Nick Suplina, managing director of law and policy at Everytown, and a former prosecutor from New York’s Attorney General’s office. “That item is marketed as a receiver, it’s marketed to be used as part of an operable firearm, and we think it should be called a firearm under the statute.”

It’s not clear who the main beneficiaries of ATF’s position on ghost guns are, apart from people who don’t want the government to know they have one. Ghost guns create a logistical headache for law enforcement, federal and local.

Members of Congress introduced two federal bills this year designed to regulate ghost guns, but both pieces of legislation stalled.