Los Angeles authorities have confirmed that a teen who killed two students, and himself, in a mass shooting at Saugus High School in Santa Clarita, Calif., on Nov. 14 was armed with a “ghost gun,” a weapon which is becoming more popular in spite of the state’s tough gun laws.

“[The Saugus High shooter’s gun] was assembled from parts, it had no serial number — it becomes what is known as a ‘ghost gun’,” Los Angeles County Sheriff Alex Villanueva told ABC7 on Thursday of the .45 caliber semiautomatic handgun Nathaniel Tennosuke Berhow, 16, reportedly used in the shooting.

Authorities don’t know how Berhow was able to get the gun, as he was too young to purchase one legally.

According to experts and law enforcement officials, ghost guns are becoming a more common — and increasingly concerning — way for criminals to bypass gun laws and get their hands on firearms.

Ghost guns are self-assembled firearms that are built from unregulated kits or 3D printers. Kits sold online or by sellers, who usually don’t have a government license, contain unfinished “receivers” or “frames.”

A receiver or frame is the main part of a gun that holds the firing mechanism. Under federal law, a completed receiver or frame is considered a gun even if it doesn’t have a trigger or a barrel. Receivers and frames that are manufactured under federal regulations are usually given a serial number, which would make a purchaser subject to a background check.

“Ghost gun” sellers, however, avoid breaking federal law by selling almost completed receivers — known as “80% receivers” — to buyers who can complete the piece on their own with tools or machinery. Some companies will sell the “80% receiver” along with the other parts necessary to fully complete a weapon; others sell the receivers on their own, leaving the buyer to purchase the remaining parts needed for their gun separately.

Ghost guns sellers operate openly online across the United States. As the Associated Press reports, “build parties” are not uncommon, events promoted on social media where people construct firearms together.

Since ghost guns are put together outside of licensed gun manufacturing systems and don’t have serial numbers, they are almost impossible for law enforcement to trace.

“These ghost guns completely circumvent our background check system and other federal and state laws meant to keep firearms from prohibited people,” Nick Suplina, the Managing Director for Law & Policy at Everytown for Gun Safety tells TIME.

David Chipman, a former Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) agent and senior policy advisor at the Giffords Law Center to Prevent Gun Violence, describes them as “an existential threat.”

“It’s almost like a build-your-own-pizza kit or buying some craft kit,” Chipman tells TIME. “[The kits] are being marketed as all of the things you need to build your own gun. Some of these sites say you can build it in 15 minutes.”

According to Chipman, the most popular “ghost gun” is built from the AR-15’s receiver.

Given their very nature, there are no comprehensive statistics on how many ghost guns are being made, or used, across the U.S. — or how many law enforcement has recovered. According to the Los Angeles Times, the Glendale, Calif. branch of the ATF said in May 2019 that 30% of all firearms that they had seized were ghost guns.

This is a figure mirrored statewide, The Trace also reported in May 2019: “According to the ATF,” The Trace said, “30 percent of all guns now recovered by agents [in California] are unserialized.”

Rudy Escalante, Chair of the California Police Chief Association’s firearms committee, says that law enforcement officials in the state are seeing an uptick in the untraceable guns’ impact on gun violence. “Ghost guns are starting to appear more frequently and being used in high-profile events,” Escalante tells TIME of the Saugus High shooting. “We are seeing them more frequently in searches, and [in instances] when we confiscate firearms.”

The Trace also reported that Calif. police departments view the rise of ghost guns as “startling.” Police departments across California tracking unserialized weapons have begun reporting recoveries in far greater number, year on year. (The Oceanside, Calif. police department recovered 19 ghost guns in 2018, per The Trace — a 280 percent rise on the previous year. In San Jose, the increase was 940 percent, with 52 guns recovered in 2018 versus just 5 in 2017.)

By the time of publication, TIME had not received a response from ATF bureaus in California as to any relevant ghost gun statistics being tracked.

In California, it is legal for someone to make their own firearm but they must, among other requirements, apply to the Department of Justice for a serial number. But this step is something that experts say criminals ignore, and are rarely prosecuted for.

Under federal law, “ghost guns” are not illegal per se. A person does not need a license to make a weapon for personal use — only when manufacturing and selling weapons is a license required. Experts say this is viewed as a loophole exploited by sellers breaking their weapons down into parts.

In 2013, a man in Santa Monica, Calif. killed five people and himself near and on the campus of Santa Monica College. The suspect had previously failed a background check and then assembled an AR-15 himself.

In 2017, a man went on a shooting rampage in Tehama County, Calif., using two AR-15-style assault rifles that he put together himself after purchasing parts online. The shooting left five people dead, including the shooter, and 18 others injured.

And in February of this year, a Texas man, Eric McGinnis, was sentenced to eight years in prison after he was caught in Dallas with a partially 3D-printed AR-15-style assault rifle and a list of lawmakers’ addresses.

McGinnis was under a court order that prohibited him from owning a firearm. He had used a 3D printer to make the “lower receiver” part of the weapon and was able to get a stock, barrel, upper receiver and grip separately.

While ghost guns are hard to trace, experts do believe there are some legislative solutions.

Chipman says that both federal and state law enforcement needs to crack down on the sale of receivers and frames, so they are restricted solely for purchase in gun stores. This would, in turn, mean that sales would include the same required legal paperwork and background checks people undertake to buy a legal gun whole.

Suplina points to New Jersey, which in November 2018 banned unregistered or unregulated gun parts, and says that other states should follow this. As the bill is only a year old, the state has not provided any statistics on its impact, but Suplina anticipates it is making a difference.

“I imagine as the understanding of how dangerous these firearms grow, you’ll see more states taking action,” Suplina says.

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Write to Josiah Bates at josiah.bates@time.com.