Lieutenant Lanny Edwards was heading in for a routine day of work when news of a shooting crackled over his radio. He quickly rerouted to a house with a terracotta roof on Holton Court, behind the Diablo Hills Golf Course in Walnut Creek, California. Once there, the first thing he saw was the yellow tape. The second thing was the body on the lawn.

Walnut Creek is an affluent community a few miles east of Oakland that sees one murder per year, if that, Edwards told The Trace. So he vividly remembers the morning of July 21, 2015, when Scott Bertics, a former Stanford University engineering student, fatally shot his ex-girlfriend Clare Orton before killing herself in her front yard.

Two guns were recovered at the scene. Normally, Edwards and his detectives begin an investigation with a firearms trace, but this case was different. When they searched the guns for their serial numbers, they found nothing. Police sometimes recover guns whose serial numbers have been scratched off, but these guns had no identifying markings at all. They appeared to be brand new, yet they were untraceable.

“My first thought, because it was a Stanford student, was, ‘Oh, my God, we have some brilliant person who’s created a way around the gun laws,’” recalled Edwards, who hadn’t seen this kind of weapon before. The detectives quickly figured out that Bertics had made the firearms himself, assembling them from separate parts he had ordered discreetly online.

The Bureau of Alcohol, Tobacco and Firearms (ATF) refers to such guns as unfinished receivers, though they’re also called 80 percent receivers, home-built firearms, or ghost guns. The existence of individual ghost guns is usually completely unknown to law enforcement, unless one turns up at a crime scene.

“Every gun tells a story,” Edwards said. “With a ghost gun, it’s just a piece of metal. There’s no way to track it back.”

Homemade guns have long been the pastime of gunsmiths who customize firearms to understand their mechanics or to build their dream rifle. More recently, libertarian provocateurs have used technology to make DIY guns as an ideological statement. One of these is Cody Wilson, a gun-rights activist and self-described anarchist. Wilson is the founder and director of Defense Distributed, a tech and gun rights nonprofit based in Austin. He first gained notoriety in 2013 with a free open-source software that allows people to 3D-print complete lower receivers out of plastic.

Last year, Wilson watched with amusement as a California state senator tried to pass a “ghost gun ban.” The senator was unsuccessful, but that new term, “ghost gun,” gave Wilson an idea. “That’s such a trick,” he thought. If gun safety advocates were trying to make untraceable guns seem scary, Wilson would do them one better. He would make it possible for almost anyone to build a precise firearm at home.

Inspired, he and his team designed and built the Ghost Gunner, a computer-numerically-controlled (CNC) mill that carves lower receivers out of metal.

“We kind of amplified and extremified it… and turned it into a product that gave it back to them,” he said.

An upgrade from the 3-D print method, which produces plastic receivers, the Ghost Gunner can mill perfect receivers out of aluminum over and over again.

“It’s very, very cheap now to make these things for yourself, just because of where we are with the state of technology,” Wilson said.

Since last April, he claims to have filled more than 1,000 orders for his machines from all over the country. He describes his customers as mostly “your red state conservative … white men, 40 to 60” years old, with about $2,000 to spare — particularly those uninterested in the government “knowing what they’ve got and how much they’ve got of it.”

While Wilson ships his Ghost Gunners, ghost guns are turning up in violent crimes, particularly in California. In Santa Monica in 2013, a young man went on a shooting spree, killing five people and then himself. A year later, gunmen took several women hostage at a bank in Stockton, which ended in three deaths. Then came this summer’s murder-suicide of Bertics and Orton in Walnut Creek. In all three cases, the shooters used weapons fashioned from unserialized lower receivers.

“The fact that they’re being sold without background checks or any of the normal documentation that’s used in lawful transactions — it’s a recipe for disaster,” said Graham Barlowe, resident agent in charge at the ATF’s Sacramento Field Office.

He started seeing crimes involving untraceable guns about two years ago, when the demand for assault weapons and ammunition soared after the Newtown massacre. Federal law allows people to build their own unregistered gun for personal use, as long as they don’t sell or transfer it to another party.

Earlier this month, Barlowe’s undercover agents arrested eight men for manufacturing and selling dozens of illegal firearms. More than 230 illegal firearms were seized, and about 90 of them were unserialized.

The agents have also discovered several other unlicensed firearms-making businesses operating out of machine shops in Sacramento, where they’ve seen electronic mills carve a complete receiver in 12 minutes.

“It is one of the biggest problems in Northern California for our office, if not the biggest problem,” said Barlowe, who estimated that his office has seized about 500 unserialized receivers since 2013.

There’s concern that untraceable, home-built firearms may mark “the next frontier of illegal firearm trafficking,” in the words of Attorney General Eric Schneiderman of New York State.

“The people who want these guns are the people who cannot buy guns at legitimate gun stores or gun shows because they have criminal background or other disqualifying factors,” he said.

The Santa Monica shooter, John Zawahri, had a documented history of mental health issues, which barred him from legally buying a gun, according to the lead detective on the case. The gun he used to kill himself and five others was made from firearm parts he purchased online.

So far, the few attempts to regulate unfinished receivers haven’t gone very far. California State Senator Kevin de León, a Democrat from Los Angeles, tried to tighten restrictions on unregistered gun parts, but the bill was vetoed by Governor Jerry Brown. Barlowe, meanwhile, says his team is “using all techniques at our disposal to identify, build a case against, and prosecute” unlicensed manufacturers, including deploying confidential informants and undercover agents, and keeping in touch with local police to catch word of any recoveries.

Of course, by their very nature, ghost guns leave agents like Barlowe unsure of how much progress they’re making.

“We have no way of knowing what’s out there.”

[Photo: Flickr user Mitch Barrie]