AUSTIN, Texas (Reuters) - “Ghost guns” like the one a 16-year-old boy used to kill two classmates and injure three others at a California high school last week are self-assembled, virtually untraceable - and completely legal.

FILE PHOTO: Police and emergency vehicles on the scene of a shooting at Saugus high school in Santa Clarita, California, U.S., November 14, 2019 in this screenshot taken from video footage courtesy of NBCLA. NBCLA via REUTERS/File Photo

The Los Angeles County Sheriff’s department confirmed that the .45-caliber pistol that Nathaniel Berhow used in the shooting at Saugus High School in Santa Clarita, California, on his 16th birthday was made from a kit. He then shot himself and died a day later in the hospital.

Such firearms have no serial numbers, and by making the gun themselves, owners can legally bypass background checks and registration regulations. That’s why they are known as “ghost guns.”

Kits can be purchased online or at gun shows, as long as the frames are not fully functional. But users can easily and cheaply machine and assemble them.

Police do not know how Berhow got his hands on the pistol he used, or who sold it and assembled it.

Kit guns represent what law enforcement and gun safety advocates call the next frontier of the fight to keep weapons away from potential criminals.

“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,” Los Angeles County Sheriff Alex Villanueva told KABC TV on Thursday in confirming the weapon used in the high school shooting on Nov. 14.

The Bureau of Alcohol, Tobacco, Firearms and Explosives did not immediately reply to questions about whether it tracks how many such untraceable weapons it recovers.

MORE UNKNOWN

“More is unknown about ghost guns than known,” said Nick Suplina, managing director for law and policy at gun safety advocacy group Everytown.

“Law enforcement is increasingly having to familiarize themselves with them, but it’s not hit the public consciousness yet that there is a legal, untraceable firearm out there that can be ordered in parts online and assembled at home.”

Suplina, a former New York state prosecutor who has worked on cases involving such guns, said law enforcement agencies have no reporting requirements for ghost guns used in crimes.

But in the past decade they have gone from relatively complex and difficult weapons to put together to incredibly simple.

To stay within federal law, the frames or “receivers” of such guns can be sold 80% complete. The other components required to build a functioning firearm are often sold along with the frame and packaged as a kit.

Kit guns vary widely in prices, like fully assembled weapons, but the same models are generally the same price.

Also included are drill bits and jigs that allow the purchaser to easily mill the frame with a simple drill press that can cost less than $100.

In recent years, federal courts convicted several people for manufacturing untraceable weapons without a license.

“Criminal enterprises and gangs are seeing a real opportunity here to mass manufacture untraceable firearms and sell them at a premium,” Suplina said.