Unregistered and untraceable, according to the federal government "ghost guns" could be the next greatest threat to U.S. security.
Ghost guns are semi-automatic handguns and rifles assembled from parts. Creating them takes no special knowledge. They can be made with some ordinary shop tools and instruction from any one of several videos available online. The result is a fully operational weapon, as high quality and accurate as guns bought in stores.
"There was no difference in accuracy and they were both able to get rounds down range," ATF special agent Geoff Rice said while firing an assault-style rifle built by a fellow agent.
Parts used to build a rifle like Rice's can be bought online and at stores like Guns Unknown in Oceanside without a background check or a photo ID.
"We're like the best, not a gun shop, in a sense," Brendon Von, manager of Guns Unknown, said.
La Mesa Attorney Scott McMillan said he builds his own weapons.
"It looked like it would be a lot of fun," he said.
According to Von, most of his customers are members of the military, law enforcement officers and enthusiasts like McMillan.
"What you are getting out of it is the satisfaction of learning how to put together your own firearm," McMillan said.
These guns are not tracked by the federal government because ATF doesn't monitor gun parts purchases. It means, guns made from these parts aren't serialized and currently don't appear on any firearms registry.
"That's this industry's dirty secret," ATF Los Angeles Field Division Counsel Paul Ware said.
By January 2019, ghost gun owners in California are required by law to register their weapons with a serial number or identifying mark. This law doesn't prevent the sale of gun parts or unfinished receivers though.
Of all the parts on the popular semi-automatic AR-15 rifle, only the lower receiver, the frame of the weapon, is required to have a serial number so it can be traced back to the manufacturer.
Ghost guns are made with unfinished receivers that don't require a serial number but do require some modification. Purchasers must perform their own finishing work in order to make the receiver usable. Using a drill, the "80 percent receiver" can be completed by milling the fire control pocket.
Such was the case with gunman John Zawahri, whose shooting spree at Santa Monica College in June 2013 killed five people and injured four others.
Debra Fine survived the massacre.
"He got my left shoulder and my right shoulder," Fine said. "(A bullet) took half of my ear then sideways through my chest."
According to ATF, because of a psychiatric disorder, Zawahri was not permitted to own a gun and without any federal laws prohibiting making firearms for personal use, he made his own.
In a security photo, released by Santa Monica police, Zawahri can be seen holding his ghost gun made from legally purchased parts and a milled lower receiver.
"I would have thought that if someone bought 1,300 rounds of ammunition online, that plus a Kevlar vest and pieces of a rifle might be a trigger or a red flag," Fine said.
The Santa Monica massacre is the only one in recent history in which a ghost gun was used.
Von said he does not sell to everyone.
"If somebody comes in and I get any type of wind that they are prohibited from owning a firearm, I will not sell them parts," he said.
According to ATF, there is no way of knowing how many ghost guns are actually out there.
"If we have concerns about terrorist cells coming into this country; if we have concerns about aliens illegally entering this country with intent to cause havoc; how can we ignore something that is so obvious underneath our noses?" Ware said.