2 Investigates: Untraceable guns showing up at crime scenes more often

SACRAMENTO, Calif. (KTVU) - Untraceable, homemade guns are showing up at crime scenes across California more often, according to federal authorities. And the guns, which are made with a particular "unfinished" part, create a dead end for investigators who are unable to trace the weapons' histories.

The part of a gun that makes it a firearm is called a receiver. It's a block of metal or plastic that can be already drilled and assembled, or as a "blank" that a buyer can drill themselves.

The receiver is usually where the manufacturer's name and a serial number are located.

"There are a lot of people who are law-abiding citizens who have an interest or hobby manufacturing guns for their own personal use," said Graham Barlowe, Resident Agent-in-Charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives Sacramento field office. "Home built firearms are perfectly legal."

Purchasing an unfinished receiver is also legal. But what is against the law is when people start using unfinished receivers to make and sell guns for multiple buyers, without requiring background checks or maintaining any paper record of the sales. According to federal authorities, that manufacturer is then essentially acting as a firearms dealer without a license.

ATF agents say they are seeing more guns made from unfinished receivers on the streets and showing up at crime scenes.

In July, gunmen used an AK-47-style rifle made from unfinished receiver to take three hostages during a Stockton bank robbery. They led police on a chase and shootout that lasted nearly an hour and ended with the death of a hostage.

"This was one of the worst crimes we'd seen here in the city of Stockton," said Stockton Police Chief Eric Jones. "Just a horrible, horrible crime and for it to be a weapon we can't trace back - I say that's a problem."

Because the weapon was made with an unfinished receiver, it became a dead end for investigators hoping to trace how the gun ended up in the hands of the bank robbers.

Federal agents believe many buyers of illegally manufactured guns would not be able to purchase a weapon legally. Police say the gunman accused in the Stockton robbery had a criminal record and would have been barred from buying a gun at a store.

In Santa Monica in 2013, investigators say John Zawahri, 23, assembled an AR-15 from an unfinished receiver and killed five people before police killed him. Zawahri had a history of mental illness and had been denied a request to buy a gun from a dealer.

Barlowe estimates that there could be half a million guns in California made from unfinished receivers. Although he says that number is only "based on rough estimates and what we know about the manufacturers."

Undercover ATF agents busted a network of sellers in the Sacramento area earlier this year. According to an affidavit, brother Emiliano and Luis Cortez-Garcia were accused of taking customers into the back room of their gun store to assemble the firearms in exchange for cash.

Law enforcement says it's a crime that's hard to prosecute, because investigators have to show that the people making the homemade guns actually intended to sell them illegally.

"If you're doing everything right and it's legal, you've got nothing to worry about," said Mark Halcon, owner of American Shooting Center in San Diego. "If you're buying a dozen of them, building a dozen of them and selling them to your friends, you got a problem."