They're a growing nightmare for police trying to investigate crimes - untraceable guns. And they're legal.
There’s a loophole allowing these so-called ghost guns to wind up in the wrong hands in San Diego.And a growing number of criminals are taking advantage.
People who buy guns from Kearny Mesa's American Shooting Center can't just walk out with them.
"We have a 10 day waiting period where they do a background check, not only on you but on the gun to make sure it's not stolen," said Marc Halcon, the store's owner.
But those rules don't apply to every gun held in San Diego, because of a loophole for people who build their own.
People can buy what are called 80 percent receivers, essentially an unfinished part used for semi-automatic rifles and other guns. The ATF doesn't consider them firearms. That means no background check and no serial number.
"If you don't have a serial number, then you don't know where the gun came from," said James Ness, a law enforcement consultant.
Many times, gun enthusiasts buy the 80 percent receivers to build their own firearms. But a new intelligence report going to law enforcement agencies says that's not always the case.
Now, criminals are taking advantage of the lack of oversight, and experts worry that could mean untraceable ghost guns going to lone wolf terrorists.
The number of ghost guns is also growing in San Diego. A spokeswoman for ATF told 10News the agency has seen a spike since 2014.
San Diego Police say they haven't received the report. Earlier this year -- governor jerry brown signed a bill requiring people who build guns from these 80 percenters to register them and get a serial number. That law takes full effect in 2019.
Halcon said the report's findings are a worry.
"It's a concern to me that anybody misuse a firearm or break the law," he said.
But he added criminals wouldn't be following the laws anyway.