"They're sold as a kit," Villanueva told KABC-TV. "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. … That is very dangerous."
Authorities in Los Angeles have noted an increase in recent years in the use and proliferation of ghost guns. The guns are crafted from parts which require no background checks to purchase and so they lack serial numbers, officials with the Los Angeles Police Department and the Bureau of Alcohol, Tobacco, Firearms and Explosives said.
Officials with the ATF are assisting Los Angeles County sheriff's deputies as they work to trace the gun used by Berhow and several other registered and unregistered firearms seized last week from his home, according to KNBC and KCBS-TV. Six of the seized guns belonged to Berhow's late father, according to KCBS-TV.
The motive behind last week's shooting remained unclear Friday, despite a search of Berhow's home and police interviews with 45 people. Authorities said Berhow had shown no signs of violence and didn’t appear to be linked to any ideology or terrorist group. He ran cross country, was a Boy Scout and had a girlfriend.
Villanueva told KABC-TV that authorities were working to unlock Berhow's cellphone in an effort to gain more information.
The Associated Press contributed to this report.