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The 16-year-old who killed two students and injured three others in a shooting at a California high school last week used a “ghost gun,” Los Angeles County Sheriff Alex Villanueva told CNN affiliate KABC.

Villanueva said the firearm was a “kit gun,” meaning it was assembled from separately acquired parts and had no serial number, making it untraceable to authorities.

“It becomes what’s known as a ghost gun,” he said.

These untraceable weapons can be assembled from kits bought online or at gun shows, Villanueva said, with the gun just partially assembled.

This allows buyers to sidestep the typical requirements that come with registering a firearm, including background checks, the Los Angeles Police Department and the ATF said in a news release last year.

“And then you can legally buy it, assemble the weapon yourself, and then you have a gun that is not registered and no one knows that you have it,” Villanueva said. “And that is very dangerous.”

The gunman, identified as Nathaniel Berhow, took a .45-caliber pistol out of his backpack at Saugus High School in Santa Clarita last Thursday and appeared to fire at random, authorities said, before shooting himself. He died the next day.

Authorities are investigating when the weapon was assembled, Villanueva said, and whether it was done by the gunman or his father, who died in 2017 of natural causes.

In the past, the father had six registered firearms, the sheriff told KABC. “Ultimately, at one point, all the weapons were lawfully removed from the home and he became a prohibited possessor,” he said.

Investigators found a kit gun during a search of the shooter’s home, Villanueva said.

A growing problem

Los Angeles has seen a rise in the number of ghost guns, the Los Angeles Police Department and the Bureau of Alcohol, Tobacco, Firearms and Explosives said last year. Detectives often recover them at various scenes of criminal activity.

Another ghost gun – an AR-15-type weapon – was used in a deadly shootout in Riverside in August.

A Utah State University paper published earlier this year said ghost guns are “particularly useful to individuals who are banned by police from owning traditionally purchased guns,” allowing owners to sidestep the legally required gun registration.

It’s not known how many ghost guns are in the US today, though one regional ATF office in California obtained 250 ghost guns in 2017 alone, the paper noted.

“That is one of the challenges of law enforcement today,” Villanueva told KABC, “because Congress and state legislatures enact all these crimes about gun registration. But now the gun industry is creating a way to just bypass the entire thing by creating a mechanism to manufacture weapons yourself.”

CNN’s Stella Chan, Susan Scutti and Jason Hanna contributed to this report.