SACRAMENTO (CBS SF) — Federal authorities are concerned about dangerous, untraceable firearms known as “ghost guns” being illicitly produced in machine shops without licenses or background checks.

Daniel Albert Crowninshield is also known on gun enthusiast forums by his online moniker, “Dr. Death.” He pled guilty on April 14 to using computer-controlled machines to churn out parts for semi-automatic rifles and other firearms without a license to do so.

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Crowninshield was caught on camera by ATF agents as they secretly recorded him coaching an agent posing as a customer through the steps to make an untraceable rifle.

“As soon as you hit metal, gently pull it and pierce the hole,” Crowninshield is heard explaining in one clip.

Crowninshield’s shop in Sacramento was so busy, customers were sometimes lined up outside the door, according to the undercover agent who recorded himself making four rifles with Crowninshield’s help.

In another undercover video, Crowninshield is heard saying, “If I go too fast for you, tell me to start over.”

“He created a fiction of them being the manufacturer,” explained Graham Barlowe, the resident agent in charge at the Sacramento field office of the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Federal law allows a hobbyist to make and own unregistered firearms but not to sell the guns. Crowninshield advertised his shop as a place where people could make guns in 20 minutes by pressing a few buttons on a computerized machine, with Crowninshield’s assistance.

ATF agents told KPIX 5 Crowninshield is a felon who cannot legally pass a background check to buy a gun.

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At one point, as the undercover agent buys four rifles, he is asked, “Are you building a personal army, or what?”

“Absolutely!” the agent replies. “Isn’t that what we’re here for?”

ATF’s Barlowe explained that — with cash payments, no questions asked, no background check, no serial number and no registration — the guns that left the machine shop vanished from the legal record.

ATF agents said they have no way of knowing how many Dr. Death sold, but believe it was in the hundreds at a minimum.

When asked if the people making the guns were collectors or criminals, Barlowe replied, “It’s hard to know, since there are no background checks done. Nobody knows.”

Two years ago, in Stockton, a ghost gun in the hands of bank robbers fired round after round out the window of the getaway car as it was pursued by police. A woman hostage died in the shootout, as did the felon who had got his hands on an AK-47-type ghost gun that he could not legally purchase.

ATF seized another ghost gun in a separate case, an AR-15-type gun that had been made into a machine gun. The seller of that gun was a convicted felon and gang member.

In recent months, ATF has seized hundreds of ghost guns and silencers made in other machine shops in the Sacramento area and arrested eight men.

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A bill now working its way through the state senate would require all such ghost guns to be registered with a state serial number.  Gov. Brown vetoed a similar bill in 2014.