Buying a fully assembled gun is a process subject to a host of regulations and restrictions, especially in states like California that tend to be more stringent.

But anyone can make a gun at home.

The do-it-yourself route is often favored by gun enthusiasts, including hobbyists and competitive shooters. It can also be a path to gun ownership for felons and people with mental illnesses or who have been convicted of domestic violence.

The Giffords Law Center to Prevent Gun Violence, part of the gun safety group founded by Gabrielle Giffords, the former U.S. representative from Arizona who survived an assassination attempt in 2011, is calling on two internet service providers to disable websites that sell materials and tools to create homemade weapons. Such weapons, often referred to as ghost guns, were used in a mass shooting in Northern California this month.

The websites, ghostgunner.net and ghostguns.com, allow customers to bypass background checks and build unregistered firearms without serial numbers.

The guns are legal as long as they are intended for personal use, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives. Individuals cannot sell or distribute firearms unless they are licensed by the bureau.

Because homemade guns cannot be tracked, it is hard to know just how prevalent they are.

Even so, in the United States, there has been “a growing number of cases involving homemade guns, some of which were high-profile active shootings by precisely the sort of people who are prohibited from buying the weapons,” Mark A. Tallman, who teaches at Colorado State University-Pueblo’s Center for the Study of Homeland Security, said in an email.

It is a problem in other countries, too.

“In foreign jurisdictions with stricter controls, homemade guns are quickly becoming a more pressing problem,” said Tallman, who has spent the last several years studying tracing data and DIY weapons. “In Australia for example, homemade guns are now conservatively estimated at 10-20 percent of illegal weapons seized by police.”

Kyle Martin, the president of Ghost Guns, said in an email Sunday that his company follows all state and federal laws.

“We deal with a lot of customers that are hobbyists,” Martin wrote. “The customers we have come into contact with usually already own firearms, have the mechanical skills to complete the manufacturing process and come from all walks of life, including many military and law enforcement.”

A spokesman for DreamHost, which hosts ghostguns.com, said in an email Saturday that the legality of the website’s content was being reviewed.

Shopify, the web host for ghostgunner.net, would not comment on individual merchants, a spokeswoman said in an email Saturday, but will “investigate material reported to us and take action” if a merchant violated its policies.

Ghost Gunner, which sells various gun components and machines used for gunsmithing, did not immediately respond to a request for comment. Cody Wilson, who runs the website, told The Associated Press that the company’s products comply with federal regulations.

“This is an attempt to apply pressure to deplatform a legal, American business selling legal products to law-abiding customers,” he said.

Ghost Gunner and Ghost Guns said they take precautions to make sure their products do not end up in the wrong hands.

“We only ship to shipping addresses that match billing addresses along with a signed receipt of delivery so the packages are not just sitting on doorsteps,” Martin said.

But without any traceable information on the various gun parts — in particular, a part called the receiver — it is difficult to know where they will end up. Although completed lower receivers are regulated by the ATF just like a fully assembled gun, the blank metal castings that can eventually be converted into a lower receiver are not.

Ghost Guns is one of several websites that sell the unmarked receivers, often referred to as 80 percent lower receivers, as well as related parts that allow customers to build an “unregistered weapon system that’s ready for almost any combat scenario,” according to the company’s website.

In 2015, eight men in California were indicted on a charge of manufacturing and selling firearms without a license. Many of the guns did not have any identification markings and were made from blank lower receivers. A total of 238 firearms and silencers were recovered by ATF agents.

In 2013, Michael Yarbrough of Corpus Christi, Texas, was sentenced to 10 years in prison for buying more than 900 parts kits and firearm receivers and selling AK-47 type firearms for transport to Mexico.

In the United States, Tallman said, “conservatively speaking, based on the stated sales numbers that some suppliers have released, there are hundreds of thousands of unmarked receivers that have been sold.”

Criminals are not necessarily ordering their receivers online, however.

“The next phase is to ask the question, ‘What can we do about more sophisticated crime groups building everything from scratch without relying on legal suppliers of components?’” Tallman added.

Neither Ghost Gunner nor Ghost Guns has been linked with criminal activity, but the Giffords Law Center argued that they were engaging in dangerous conduct and facilitating unlawful activity.

The two companies warrant attention because their marketing focuses “on the anonymous, untraceable nature of the guns they allow to be made — and the many examples of ghost guns used in crimes,” Adam Skaggs, chief counsel at the center, said in an email.

Kevin J. Neal, the gunman in Northern California who killed five people and injured 12 this month before he was killed by police, was prohibited from having firearms because of a restraining order. He made his own semi-automatic weapons at home.

“I think this is going to be a growing problem, especially with the advent of 3-D printed guns,” Adam Winkler, a gun policy expert and law professor at the University of California, Los Angeles, said in a phone interview.

An expanded assault weapons ban in California bars residents from owning or selling AR-15s equipped with bullet buttons, devices used to quickly remove the ammunition magazine, which means that making this type of gun at home will be illegal, too.

California residents who lawfully possessed such an assault weapon before 2017 can keep it as long as the weapon is registered. That prompted a surge in gun sales in 2016.

“When you have prohibition or quasi-prohibition, that’s the lever that pushes up homemade production,” said David Kopel, the research director of the Independence Institute, a libertarian think tank, who is also an adjunct professor of law at the University of Denver.

“We’re now in a phase where that artisanal manufacture is going to become more important.”