Ghost guns growing in popularity as law enforcement struggles to trace the unregistered weapons

At Guns, Fishing and Other Stuff in Dublin, the owner says candidly why there is no place for ghost guns, unregistered guns made from different parts.

"They were designed for criminals. That's the best way to say it," said Travis Morgan.

The Los Angeles County sheriff revealed Thursday that the .45 caliber handgun a teenager used to kill two fellow high school students and himself in Santa Clarita last week was a ghost gun.

Ghost guns have no serial numbers. There's no way authorities can trace them.

The Alameda County sheriff's department showed KTVU pictures of dozens of ghost guns they have taken off the streets in the past year.

"Organized guns and organized narcoitcs traffickers. That's where we see ghost guns," said Sgt. Ray Kelly of the Alameda County sheriff's department.

The U.S. Alcohol, Tobacco and Firearms estimates 30% of the guns it seizes in California are ghost guns.

"They're really common. There are a lot of them out there," said Kelly.

Gunmen in Stockton used a ghost gun hand assembled to kill an innocent bystander during a bank robbery in 2014.

Authorities say they have become even more common since then.

The gun parts can be purchased online, or at some gunshows, and even at manufactured with a 3-D printer. 

There are no background checks or paperwork. Other than drilling a few holes, there's very little to do to a ghost gun to get it ready to fire.

"What is classified as the gun is the receiver. So you can have a ghost gun receiver and have after market parts  and have them shipped to your home," said Kelly.

"If you've got all the parts it will take you 2-3 hours to build the entire gun," said Morgan.

California instituted a law last year requiring owners to register their homemade guns with the state, and get serial numbers.

"If you want to break the law it is easy to do. Becasue nobody knows you got it," said Morgan.

California's law did not stop the 16-year-old Santa Clarita student from using a ghost gun to kill two.