School personnel are seen on campus at Saugus High School on November 15, 2019 in Santa Clarita. (Photo by Frederic J. Brown / AFP / Getty Images)

This story is published in partnership with The Trace, a nonprofit newsroom covering gun violence.

Updated Nov. 21 at 12:30 p.m.

The weapon used in the Santa Clarita, California, school shooting was a homemade, unserialized pistol, otherwise known as a "ghost gun."

The Los Angeles County Sheriff's Department confirmed on Thursday that the .45-caliber handgun was crudely assembled from parts available for purchase without a background check. Officials said they had no indication who purchased the parts for the firearm, or who assembled it.

The Trace and KPCC/LAist first reported that law enforcement were investigating whether or not the gunman had used a homemade weapon. It was confirmed by the Los Angeles Times.

Authorities identified the gunman as a 16-year-old Saugus High School student. They said he used the handgun to shoot five classmates, killing a 14-year-old boy and 16-year-old-girl. Authorities said no rounds were left in the gun's chamber.

EXPLAINER: 'Ghost Guns' Are Becoming A Big Problem In California. Here's Why.

The gunfire lasted just 16 seconds and prompted police to scramble throughout the campus and surrounding neighborhoods to locate the shooter. Authorities used surveillance video at the school to determine he shot himself in the head at the scene. He died Friday at a hospital.

In an interview with ABC7 on Monday, Sheriff Alex Villanueva said authorities had found at least one additional homemade weapon at the shooter's residence.

He also addressed weapons owned by the shooter's father who died in 2017. The father's obituary noted he was an avid hunter and featured images of him with weapons. Villanueva told ABC7 the father at one time had six guns registered to him.

"All the weapons were lawfully removed from the home and he became a prohibited possessor," he said. "The guns were subsequently destroyed, legally."

It was not clear why the father had been prohibited from owning firearms. He had convictions for two DUI offenses on his record.

As The Trace reported in May, more and more homemade, unserialized weapons are popping up at crime scenes across California. Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives now say that nearly a third of the guns recovered by the agency in the state are homemade.

Ghost guns provide a host of challenges for law enforcement. Chief among them is that they enable minors or those with criminal records to acquire firearms without having to go through a background check or create a trail of paperwork surrounding a gun purchase. Because ghost guns have no serial numbers, they are almost impossible to trace.

California law enforcement agencies say that the majority of ghost guns now being used by criminals are AR-15-type rifles or Glock-type pistols. But one of the earliest models to hit the DIY gunmaking scene was the 1911 handgun. Kits to construct the pistols remain available online.

The popularity of ghost guns has been propelled by advances in technology and the proliferation of cheaper gunmaking tools, such as CNC machines and drill presses. Ghost guns are now openly sold online in near-but-not-quite complete form, colloquially referred to as 80 percent receivers (the user must finish the remaining 20 percent for it be considered a "gun" under law). Once the lower receiver is complete, the other parts, such as upper receiver and the firing controls, can be assembled within minutes, creating a fully functional, yet untraceable firearm.

These kits are free from federal regulation because of a carve out in the 1968 Gun Control Act allowing builders to manufacture weapons at home for personal use without submitting to a background check. However, five states, including California, have passed laws designed to rein in this nascent market.

Homemade weapons are rapidly becoming the tool of trade for criminals who'd be otherwise barred from purchasing guns through more conventional means. In California that has meant some sharp increases in ghost guns recovered by police departments:

San Diego Police Department saw a jump from five ghost guns recovered in 2017, to 52 the following year.

San Francisco Police Department recovered 15 homemade firearms in 2017 compared to 44 in 2018.

The Los Angeles Police Department does not track recoveries of ghost guns.

Ghost Guns have showed up in a variety of high-profile shootings in California, including two other mass shootings. In 2013, after failing a background check at a gun store, a 23-year-old lone gunman used a homemade AR-15 rifle to kill five people at Santa Monica College. The weapon was illegal to possess under California law.

In 2017, a 44-year-old man with a criminal record that barred him from purchasing a gun assembled two AR-15s with parts bought online. The man used the guns to go on a 25-minute shooting spree that left five dead and 18 injured.

Ghost guns are increasingly being turned against police officers as well. In August, a roadside gun battle in Riverside left a California Highway Patrol officer dead, and two others wounded.

This has prompted the state of California to beef up controls on ghost gun ownership. In 2016, lawmakers passed a bill requiring each homemade weapon in the state to have a serial number and be registered with the California Department of Justice. A new law, signed in October by Governor Gavin Newsom, also requires background checks for individuals seeking to buy firearm parts.

Meanwhile, two bills designed to regulate ghost gun sales and parts are pending in Congress.

LAist/KPCC reporter Elly Yu contributed to this report.

@AlainStephens can be reached on Twitter.

12:30 p.m.: This article was updated to indicate that LASD confirmed the suspect used a ghost gun. This article previously reported the father's weapons had been recovered at the family home, based on statements last week by sheriff's officials.

This article was originally published at 3:45 p.m. on 11/20/19.