A legal loophole means that anyone, including criminals, can order a so-called "ghost gun" off the web without a background check - a gun with no serial number that can't be traced.
The guns are built from kits and arrive in pieces, so under existing law, when they're shipped, they aren't guns. When assembled by their buyers, they're lethal - and legal.
Federal officials like Graham Barlowe, the resident agent in charge of the ATF's Sacramento office, say the loophole is dangerous.
"People that could not pass a background check," said Barlowe, "are purchasing these unfinished receiver kits and making firearms because they know that if they went to a gun store, they wouldn't be able to pass a background check."
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Police say criminals are well aware of the availability of "ghost guns," and they've been used in shootings across the country, from Maryland to California.
Jeff Rossen, NBC News national investigative correspondent, went online to see how easy it would be to order these gun kits. He quickly found dozens of websites offering the product, and ordered a rifle kit, which he had shipped to former ATF agent Rick Vasquez in Virginia.
All the parts needed to assemble a gun were in the box when it arrived. It took Vasquez a couple of hours to assemble the weapon.
"This is now a completed semi-automatic firearm," said Vasquez, showing it to Rossen. Rossen noted that there was no serial number on the finished product, making it untraceable.
Said Vasquez, "That is correct ... You cannot trace this firearm." He and Rossen then took the weapon to a range and fired it, where in Vasquez's expert opinion, it "work[ed] great."
Sen. Chuck Schumer, D.-N.Y., the senate minority leader said he wants to close the loophole, but expects fierce opposition from the National Rifle Association. "We are going to try to pass legislation," said Schumer. "The trouble is the NRA is so unreasonable and has such power in the Congress, you'd think this should pass like that, but it is going to be a long hard road."
The NRA did not respond to repeated requests for comment.