A teen who fatally shot two students and injured three others in Santa Clarita, California, used an unregistered "kit gun" in last week's shooting, authorities said.
Kit guns, sometimes called "ghost guns" if they do not have a serial number, are self-assembled firearms, according to the Giffords Law Center to Prevent Gun Violence.
Los Angeles County Sheriff Alex Villanueva told KABC-TV that the gun used in the shooting at Saugus High School does not have a serial number to track.
"The real concern here is that you have untraceable, unserialized firearms that exist completely outside of the regulatory scheme of federal and state law," Nick Suplina, managing director for law and policy at gun violence group Everytown, told USA TODAY. "People who are prohibited from owning firearms under federal or state law have ready access to make their own untraceable firearms, and that's very dangerous."
'Ghost gun' used:Santa Clarita gunman used unregistered 'ghost gun' in Saugus High School shooting, sheriff says
Are ghost guns more dangerous than other firearms?
Ghost guns are untraceable, can be made from plastic parts and do not require a background check, according to the Giffords Law Center to Prevent Gun Violence.
Gunmakers or importers affix a serial number and markings to traditionally manufactured firearms that identify the manufacturer or importer, make, model and caliber, the center said.
The Bureau of Alcohol, Tobacco, Firearms and Explosives can trace these guns to their first retail purchaser when investigating a crime.
Those who make kit guns and parts can say they are not selling an actual firearm and are not required to use a serial number under federal law, which makes the chain of custody nearly impossible to track, the Giffords Law Center said.
Though some kit guns may have a serial number, Suplina said most are untraceable and, therefore, ghost guns.
Buyers of ghost guns do not face federal background checks because they are purchasing "unfinished" frames or receivers.
Suplina said the federal government considers frames or receivers – which house other parts, including the firing mechanism – the part of the gun that makes it a firearm and requires a serial number and background check to purchase. If a receiver is considered unfinished under federal purview, it is not regulated like a firearm, he said.
Some of these firearms may be made with plastic parts. This makes them undetectable to traditional metal detectors, the Giffords Law Center said.
Where do ghost guns come from?
Kit guns can be sold online or at gun shows, Villanueva told KABC-TV. Buyers can use simple tools to build the firearm, according to Newtown Action Alliance.
"They're also known as an 80% gun. So 80% of it is assembled already, and you also get the additional 20%," Villanueva told the TV station. Some are sold as a package with all the parts and tools needed to complete the firearm, Suplina said.
"You're essentially ordering a firearm, but just completely outside the rule of law," Suplina said.
Some untraceable guns may be 3D-printed. According to the Giffords Law Center, the models and printers can be purchased for about the same cost of a traditionally manufactured firearm.
Newtown Action Alliance said that in some cases, these firearms can be made almost entirely from plastic parts, except for a metal nail used as a firing pin.
Are ghost guns illegal?
Federal and some states' laws prohibit some undetectable firearms, the Giffords Law Center said.
The Undetectable Firearms Act requires that guns are detectable by metal detectors "after removal of grips, stocks, and magazines" and that X-rays can detect all major components, including "the barrel, the slide or cylinder, or the frame or receiver."
A person could make a gun with largely plastic parts and some metal parts that are easily removable before entering a security area, the Giffords center said.
The ATF determines whether a product is a firearm that requires a background check and serial number or if it is an unfinished kit and requires no check or serial number.
California passed regulations that require self-assembled firearms to have serial numbers, among other restrictions. New Jersey prohibits the sale of firearm parts that "may be readily manufactured or otherwise assembled" without a serial number. Those who want to use a 3D printer to build a gun must be a registered or licensed firearm manufacturer or dealer, the Giffords Law Center said.
How did the Santa Clarita shooter obtain one?
It is unknown who bought the pistol's parts or put it together, Los Angeles County Sheriff’s Lt. Brandon Dean said in a statement.
Authorities found several other unregistered guns in his home in Santa Clarita, near Los Angeles. They are investigating where the guns came from and noted registered guns belonging to his father were seized in 2016.
Villanueva said the 16-second shooting blitz was deliberate and the shooter seemed familiar with the weapon. The gun jammed, but he quickly cleared it.
Contributing: Kristin Lam
Follow USA TODAY's Ryan Miller on Twitter @RyanW_Miller
