(WHDH) — There is growing concern about do-it-yourself weapons that criminals can build at home.

Buy the parts online, and build a gun at home.

No background check.

No serial number.

“Here is an easy way for someone who shouldn’t have a firearm to get one,” said U.S. Rep. Jim McGovern, D-Worcester.

“It’s just clearly meant to circumvent federal law,” said Sam Rabadi, the former Special Agent in Charge for the Philadelphia Field Division of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).

They’re often called “ghost guns.” And 7NEWS discovered that they’re popping up right here in Massachusetts.

Police seized a ghost gun in Duxbury last summer. Christopher Barlow Jr., 21, was charged with various weapons crimes after police said they discovered his stockpile of firearms, ammo, and explosives. And according to a search warrant obtained by 7NEWS, Barlow built the ghost gun himself.

You can buy all of the parts you need to build a ghost gun online for about $500. That’s because even the lower receiver, the main component, is not legally a firearm until you drill additional holes in it. And experts say it can take as little as a few hours to put it all together.

In November, five people were killed in a shooting spree in northern California, which began at an elementary school. Police said the gunman was not allowed to possess a gun – so he built two semi-automatic rifles himself.

The ATF says ghost guns have been found at shooting scenes, and on gang members, across the country.

“It’s very frustrating,” Rabadi said.

He said it’s a loophole for criminals, the mentally ill, and others who normally wouldn’t be able to buy a gun.

“Now they can just go right on the internet, order the parts, and go ahead and construct it in their own garage. That’s a problem,” Rabadi said.

He also warns that guns without serial numbers hamper investigations because police can’t trace where they came from.

“It makes our jobs so much more difficult,” Rabadi said.

The ATF office in Boston said it has come across ghost guns during its investigations. Jason Guida, a local attorney who specializes in gun laws, said he gets calls from people in Massachusetts who are building them – and that it’s getting easier.

“I think as technology progresses, this will become more common,” Guida said.

“This loophole is deadly,” McGovern said.

McGovern is co-sponsoring a bill in Washington, D.C. that would classify even gun parts as firearms. That means you’d need a background check to buy them, and they’d have to have serial numbers.

“They are firearms, and it’s nuts that the ATF doesn’t classify them as firearms,” McGovern said.

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