WASHINGTON (NewsNation Now) — President Joe Biden is tightening regulations of buyers of self-assembled untraceable “ghost guns” — homemade firearms that usually are assembled from parts from gun kits and lack the serial numbers used to trace them.

Biden made the announcement Thursday, April 9, along with half-dozen executive actions aimed at tackling gun violence in the United States.

But what is a ghost gun?

So-called ghost gun kits are self-assembled from parts purchased online or at gun shows and are increasingly associated with crimes. But they are not classified as firearms and so can be legally sold without serial numbers or background checks.

For years, federal officials have brought attention to an increasing black market for homemade, military-style semi-automatic rifles and handguns. Ghost guns have increasingly turned up at crime scenes and in recent years have been turning up more and more when federal agents are purchasing guns in undercover operations from gang members and other criminals.

The critical component in building an untraceable gun is what is known as the lower receiver, a part typically made of metal or polymer. An unfinished receiver, sometimes referred to as an “80-percent receiver,” can be legally bought online with no serial numbers or other markings on it, no license required.

Without a serial number, authorities cannot trace the chain of custody from the gun, making it difficult to figure out whose gun it is. Ghost guns can also be fashioned from parts created with 3D printers, adding another roadblock when tracing the gun.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said it “enforces Federal firearms laws and, currently, these laws do not limit the technology or processes that may be used to produce firearms.”

It’s is also legal to build a gun in a home or a workshop and there is no federal requirement for a background check, meaning anyone could buy and build one.

“Modern guns are not simply cast or forged anymore, but can also be made of plastic, printed on a 3D printer, or sold in self-assembly kits,” Attorney General Merrick Garland said.

The Justice Department is expected to announce a rule requiring ghost gun kits be treated as firearms under the Gun Control Act, which would require that the parts be made with serial numbers and that buyers receive background checks. The DOJ is expected to issue a rule within 30 days.

The goal is to “help stop the proliferation of these firearms,” according to the White House.

As a part of Biden’s executive actions, he will have the AFT and Justice Department issue an annual report on firearms trafficking. The last report was issued in 2000 and does not account for things like “ghost guns” according to the Biden administration.

Some states, like California, have enacted laws in recent years requiring serial numbers to be stamped on ghost guns. The ATF has said more than 30% of the illegal weapons it has confiscated in some areas of California are ghost guns.

A gunman who killed his wife and four others in Northern California in 2017 who had been prohibited from owning firearms built his own to skirt the court order before his rampage. And in 2019, a teenager used a homemade handgun to fatally shoot two classmates and wound three others at a school in Southern California.

Biden also nominated David Chipman to be director of the ATF. Chipman is a former federal agent, adviser at the gun control group Giffords and a gun owner himself. During his time as a senior policy adviser with Giffords, he spent considerable effort pushing for greater regulation and enforcement on ghost guns, reforms of the background check system and measures to reduce the trafficking of illegal firearms.

While the House passed a background-check bill last month, gun control measures face slim prospects in an evenly-divided Senate, where Republicans remain near-unified against most proposals.