By Maria Dinzeo

SAN FRANCISCO (CN) — Fed up with the growing number of untraceable homemade firearms used in gun crimes and mass shootings, California Attorney General Xavier Becerra announced a federal lawsuit Tuesday to force the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives to crack down on so-called “ghost guns” that skirt laws requiring background checks and age verification.

Lacking a commercial serial number and purchasable without a background check, “ghost guns” are not considered firearms subject to ATF regulation under the Gun Control Act because a central piece remains unfinished — the receiver or frame of the weapon, which houses all of its internal components including the barrel and trigger mechanism.

Becerra disputes this interpretation. His lawsuit notes the Gun Control Act expressly provides that a receiver or frame can be considered a firearm because such pieces are “designed to or may readily be converted” into functional weapons.

“We are suing the ATF over its interpretation what qualifies as a firearm,” Becerra said on a call with reporters Tuesday. “ATF allows these guns to go unchecked despite having the authority under the Gun Control Act to regulate them.”

He was joined by Bryan Muehlberger, whose daughter Gracie Anne Muehlberger was shot and killed at Saugus High school in Santa Clarita, California, by a fellow student with a “ghost gun” assembled from an online kit.

“Until that tragic day, I had never even heard of a ghost gun,” Muehlberger said. “We all need to wake up and understand the danger here. Anyone, and I mean anyone, can buy these totally unregulated kits with just an internet connection and a credit card. And that’s how my daughter’s killer got his murder weapon. I still struggle to understand why it can be so easy to order a kit online that has no intended purpose other than creating a fully functioning firearm and how the ATF can allow ghost guns to be sold with no federal regulation.”

And they’re easy to put together.

“If you can assemble Ikea furniture you can probably assemble a ghost gun. And you can probably do it faster,” said Hannah Shearer, litigation director of the nonprofit legal organization Giffords Law Center.

Shearer, who is co-counsel in the lawsuit, said state gun laws have been undermined by the ATF’s refusal to treat the parts used to build homemade guns as firearms. This allows “unscrupulous” DIY firearm sellers to flood the gun market.

“The ATF has specifically exempted them from the federal definition of a firearm, meaning federal gun laws don’t apply to them. As a result, people can order and build these guns without passing background check or even verifying their age,” she said. “This has opened the biggest loophole you can imagine in our federal and state gun laws.”

Muehlberger, who is not opposed to firearms and owns guns himself, said that only a few months ago he bought a ghost gun kit online using his daughter’s name.

“Sure enough, the company mailed out the kit with Gracie’s name emblazoned on the box. Had the company done any investigation — just Googling her name — they would have learned that not only was Gracie barely 15 and no longer with us, but that she was also killed by a ghost gun,” he said. “It’s that easy for anyone, including children, those with mental illness or past issues with violence, and those that are not legally allowed to own guns, including a dead girl, to circumvent our laws and get a gun.”

The state’s lawsuit says ghost guns are becoming the “weapon of choice” for gun traffickers, gangs, and political extremists like Steven Carrillo, who shot and killed federal officer David Underwood in June outside the federal courthouse in Oakland.

The state of California demands a court order nullifying the ATF’s classification of an unfinished rifle or pistol frame as not a firearm, and a finding that its guidance on 80% receivers and frames is arbitrary and capricious under the Gun Control Act.