Law enforcement agencies are facing a new and growing problem: Homemade, nearly impossible-to-trace firearms, otherwise known as ghost guns. But there are many misconceptions about what qualifies as a ghost gun, and much confusion about the challenges they present.

Let’s break down the basics:

What exactly is a ghost gun?

The term “ghost gun” is used by media, police, and sometimes the firearms industry to describe homemade weapons devoid of serial numbers or other identifying markings that enable them to be tracked to their maker, seller, or original owner.

For years, law enforcement authorities referred to these weapons as simply “homemade” guns or “kit” guns. “Ghost gun” is a relatively new term.

How are ghost guns made?

A variety of ways.

The most important part of a gun is what’s known as the lower receiver. It’s kind of like the chassis of the weapon, housing vital components like the hammer and trigger. According to federal law, the lower receiver is actually a firearm. While you can walk into a store and purchase a new barrel or handguard for your rifle without a background check, the lower receiver is treated as a de facto weapon.

One of the most popular methods for creating a ghost gun entails buying pre-made parts and assembling them into a gun at home. There are dozens of companies that sell what are known as “80 percent” lower receivers. What does that mean? You guessed it: A lower receiver that’s 80 percent finished. With a little elbow grease, anybody can take the receiver to 100 percent. Add a few more parts — which can also be purchased without a background check — and you have a fully functional firearm.

This may sound like a lot of work, but experts say it’s not. “If you can put Ikea furniture together, you can make one of these,” said Carlos Canino, the special agent in charge of the Los Angeles Division of the federal Bureau of Firearms, Alcohol, Tobacco and Explosives (ATF).

Wait — I thought all ghost guns were 3D printed?

That’s a common misconception. The Liberator, the first 3D-printed pistol to function successfully, was created in 2013 by self-described anarchist and ghost gun guru Cody Wilson, received widespread media coverage and raised fears that such weapons could arm terrorists and bypass metal detectors. (The latter concern is mostly unfounded.) As The Trace has reported, Defense Distributed, the company that released the Liberator, continues to operate.

The practice of distributing blueprints for 3D-printed guns online is highly controversial, and has been at the center of a years-long legal battle between the Department of State and Defense Distributed, a gun advocacy and manufacturing group. In 2013, the Department of State banned the publication of blueprints on the Internet. That decision that was reversed by the Trump administration in 2018. In November of this year, a judge overruled the Trump administration, reinstating the ban. Defense Distributed plans to appeal.

A similar method to 3D-printing involves computer numeric control machines, which can carve unserialized receivers from metal. Such machines run from $2,000 to upwards of $50,000.

Yes. In fact, America has a long tradition of unregulated gunmaking.

It was only in 1968, with the passage of the Gun Control Act, that gunmakers were required to obtain a license from the federal government, and stamp serial numbers on the weapons they produce.

But the Gun Control Act provided an exemption for people who make their own firearms for personal use. And the Brady Gun Violence Prevention Act, which established the federal background check system in 1993, included a similar allowance for homemade weapons. In other words, it’s perfectly legal for Americans to build their own firearms.

You keep mentioning serial numbers. Why is it a problem when a gun doesn’t have one?

When a law enforcement agency recovers a firearm in a criminal investigation, it can submit the weapon to the ATF for what’s known as a trace. A trace report is kind of like a CarFax report for a gun: A detailed documentation of its life that includes the name of its manufacturer, distributor, seller, and purchaser. This information can be key to solving crimes. And when used in aggregate, trace data can provide insights into gun trafficking patterns.

It’s nearly impossible for the ATF to trace a gun without a serial number. Ghost guns don’t have any unique markings, and therefore present informational black holes to police investigators.

Is tracing the only reason police and lawmakers are concerned about ghost guns?

The other big issue is that these weapons exist outside the traditional supply chain for guns. There’s no paperwork showing that they exist, and they don’t require a background check.

In other words, ghost guns provide an easy avenue for people with criminal records to obtain firearms. For example, if you are banned from owning guns in a state with universal background checks, you won’t be able to walk into a store and purchase a gun. But if you order parts online and create your own ghost gun, it’s likely that nobody will know.

So are a lot of criminals using ghost guns?

It appears so.

Earlier this year, The Trace published an investigation into the rise of ghost guns in California. According to the ATF, 30 percent of all weapons recovered by the bureau in the state were homemade.

These weapons have been used in a number of mass shootings, as well as high-profile shootouts with police. Arms traffickers have also identified homemade firearms as a lucrative enterprise. It’s not exactly a surprise: In a high-regulation state like California, weapons command a premium on the black market. But an arms dealer can construct a gun with parts worth a few hundred dollars, and then hawk the weapon for four times the cost.

Still, there are serious gaps in what we know about how often ghost guns are used in crimes. When The Trace published its story on California, the ATF said it did not track ghost gun recoveries on a national scale. And many local law enforcement agencies fail to do the so in their jurisdictions. In Los Angeles County, for example, a 16-year-old gunman used a homemade handgun to kill two of his classmates this November. But the county does not track recoveries of homemade weapons.

Have there been any efforts to regulate ghost guns?

There are currently two federal bills designed to regulate ghost guns. The proposals would subject homemade weapons to the same regulations and licensing requirements as traditional firearms. Both bills are stalled in committee.

In lieu of federal controls, four states — California, Connecticut, New Jersey, and Washington — have passed their own laws. California’s are perhaps the most expansive. In 2016, state lawmakers approved a regulation requiring homemade gunmakers to serialize and register their weapons with the state. And a new bill, passed in October, would require residents to go through background checks to acquire certain firearms parts, like 80 percent lowers.

New Jersey, which passed its law in 2018, has also attempted to crack down on the proliferation of ghost guns by suing companies that manufacture the parts needed to make firearms.