L.A. County Sheriff Alex Villanueva has confirmed that the attacker at Saugus High School used a ghost gun on Nov. 14 to kill himself and two students, and injure three other students.

Ghost guns are put together with unregistered parts that can’t be traced by authorities.

“They're not considered firearms, even though they are very near complete to firearms. And for that reason, they can be sold without a background check, and they can be sold without having to conduct any sort of paperwork,” says Alain Stephens, who covers guns for The Trace. “It's essentially the legal equivalent of a paperweight. So if you can order a paperweight online, you can get these kits. So yes, teenagers can get them. Felons can get them. Prohibited people can get them.”

California law enforcement says they’re seeing an uptick in these weapons being used.

“The overwhelming majority of people that do this are hobbyists. But because there's no background check and because it's difficult for law enforcement to trace, this is something that criminals have latched onto, especially in states like California, where we have stricter gun control than the rest of the country,” says Stephens. “When I talked to authorities there, especially California law enforcement, they said that they feel like California may be thrown to the wolves.”

In September, the state of California adopted legislation that requires background checks on “precursor” gun parts that are used to assemble ghost guns. But for the most part, the guns aren’t tracked by local agencies. Stephens says that the ATF’s Los Angeles Division is one of the few agencies taking account of the weapons.

“They said about 30% of the guns they're seizing now are these homemade unserialized weapons,” Stephens says. “I think in the future, this is going to be a continuing problem.”