A troubling new gun problem is popping up in cities all over the United States: "ghost guns." "Ghost guns" can be made to look and work like any firearm and are totally untraceable.

"A 'ghost gun' is a firearm that has been manufactured from parts by a person who is not typically a licensed firearms manufacturer," said Graham Barlow, special agent in charge of the Sacramento office of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). "If they are manufactured for personal use, they are not required to be registered and they are not required to be marked. Someone could lawfully have a home-built firearm that is not registered or marked in any way."

A ghost gun is not easy to make, but anyone with the skill, money and access to certain tools can craft a deadly weapon within a few hours.

Thanks to the Internet, there is a small but growing industry of do-it-yourself gun-making. With a few clicks on the computer anyone can find companies like Guns80.com.

"Our typical customers are law-abiding U.S. citizens that are hobbyists and gun enthusiasts," said James Gray, Guns80.com.

Gray sells the necessary part to making a gun: It's called an "unfinished lower receiver."

"This is basically everything you need to assemble a rifle," said Gray. "You would receive our $550 rifle kit that we sell right now. You'd get an upper receiver, a lower receiver that is incomplete and totally unmilled. You'll drill out the holes in it, and then you'll mill it out. It sounds simple but it requires a little bit of work to do."

Gun-control advocates don't think it is a good idea to allow anyone to make an unregistered lethal weapon.

And every day more and more criminals are using ghost guns.

Mass murderer John Zawahri built his own version of an assault rifle because he did not pass a background check to buy a gun. He then went on a shooting spree in Santa Monica, California that left five people dead.

"Had that firearm been recovered at the scene and perhaps he escaped, there would have been no way to track him down," said Graham Barlow of the ATF.

And during a spectacular shootout in Stockton, California, bank robbers used fully automatic ghost guns. What's more troubling is that those weapons were obtained on the black market, pre-made with no serial numbers. It is against the law to make a ghost gun to sell to a convicted felon.

"When crimes occur and police officers, state, federal and local recover a firearm, one of the first things they look for is the serial number on that firearm and the markings identifying who the manufacturer is," said Barlow. "That information is then used to trace the firearm through the ATF tracing center and determine who the first retail purchaser of that firearm was. When they recover one of these firearms that are made from the ghost guns, there is nowhere to go with it. There are no markings. There is no serial number."

The ATF has been trying to stop the sale of ghost guns to criminals and even Mexican drug cartels.

"I have interviewed people and asked them what it was about these firearms, especially considering in many occasions you can buy a commercially made AR-15 for less than what it takes, costs to manufacture your own," said Barlow. "One of the reasons it keeps coming back time and time again is that people do not want to own a firearm traceable to them. They do not want government records kept with their names on them and they would like to remain anonymous."

In 2014, California State Senator Kevin de Leon (D-Los Angeles) introduced legislation that would require all homemade guns to be registered and the owner to complete a background check.

"The threat is real," said de Leon in 2014. "We're even beginning to see an emerging industry and market for ghost guns. They are being produced without serial numbers or records of sale. No one knows they exist until after a crime has been committed."

The bill passed in the California Legislature but was later vetoed by Governor Jerry Brown.

"I feel if anybody's trying to make a gun in secret they're up to no good and they need to be stopped," said Marianne Orton Johnson, whose daughter was killed by an ex-boyfriend who obtained a ghost gun. He later killed himself. "If they're trying to buy on illegally, legally or trying to avoid any of that scrutiny they're probably not going to just be a collector, target shooter or something. They're looking to kill."

"I don't think that question's relevant whether somebody's going to take one of our products, build a firearm and use it for criminal purposes," said James Gray, Gun80.com. "I mean, are knife-sellers concerned someone's going to buy their kitchen knife and use it for criminal purposes? I mean, is Facebook concerned someone's going to use Facebook for criminal purposes or other social media?"

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