LOS ANGELES — Business is so good at online retailer Ghost Gunner that it's sold out of its most popular items: key pieces for homemade assault-style rifles — known as "ghost guns" — which exist under the radar of law enforcement.

The "80 percent lower receiver" parts are so named because they fall just short of meeting the federal definition of being guns unto themselves.

They can easily be shaped and fastened into full-fledged killing machines. But those who want to use Ghost Gunner's popular CNC (computer numeric control) tool to do this will have to wait: it's sold out, too.

The future of firearms — 3D-printed weapons and ghost guns — have officials so alarmed they're calling for stricter regulation. Both types lack serial numbers that would enable law enforcement to track them, and both are available in plastic, which can bypass metal detectors, especially if certain parts like firing pins are removed.

The National Rifle Association says fear of plastic guns is overblown because they were outlawed from coast to coast by the Undetectable Firearms Act of 1988. Ghost guns made with metal parts purchased online, however, remain legal in most of the country.

When guns are in the news, business is good, Cody Wilson, the man behind Austin, Texas-based Defense Distributed, told NBC News in a recent interview. "Every time there’s a big national stink," he says, "we sell out."

Travis Lerol holds an AR-15 assault rifle along with a rifle's lower receiver made of plastic that was constructed by his 3D printer at his home in 2012. Jahi Chikwendiu / The Washington Post/Getty Images file

Wilson has been getting attention because the U.S. State Department recently agreed to end its prohibition of his online publication of blueprints that allow anyone to make an untraceable firearm from scratch using a 3D printer.

A federal judge on Tuesday, however, granted a request for a temporary restraining order that prevents Wilson from publishing the plans for how to make the untraceable guns.

“In a major victory for common sense and public safety, a federal judge just granted our request for a nationwide temporary restraining order — blocking the Trump administration from allowing the distribution of materials to easily 3-D print guns," said New York Attorney General Barbara D. Underwood, who was part of a coalition of nine states suing over the firearms instructions.

But a coalition of gun rights groups created a website and posted similar gun blueprints later Tuesday night, seemingly in protest against the judge's ruling.

President Donald Trump tweeted Tuesday that he is "looking into 3D Plastic Guns being sold to the public" and noted that he had "already spoke to NRA."

Attorneys general from 20 states had formally urged Secretary of State Mike Pompeo and Attorney General Jeff Sessions Monday to reverse course and ban publishing of the instructions.

Los Angeles City Attorney Mike Feuer, co-chair of the group Prosecutors Against Gun Violence, says 3D-printed guns, like ghost guns, will be embraced by felons unable to legally obtain firearms in California.

Publishing the blueprints "would allow people who can’t get guns to download them and evade detection," he said.

Separately, ghost guns — the unmarked, unregistered result of those "80 percent" parts widely available online — made headlines in July when authorities busted an alleged firearms ring in Los Angeles.

The day after Independence Day, officials in L.A. announced that a confidential informant and an undercover federal agent had purchased 20 assault rifle ghost guns from a pair of suspects who now face allegations of unlawfully manufacturing and trafficking weapons, according to a criminal complaint.

The alleged sellers, Julio Herrera, 27, and Raychelle Hernandez, 26, were part of a gang-connected ring that sold completed ghost guns to associates in Hollywood, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives and the Los Angeles Police Department.

Ghost gun parts have been available on the internet throughout most of the decade, but authorities say they're increasingly coming across gang members and felons in possession of resulting firearms. Federal law hasn't kept up with the online availability of these parts, and attempts to pass new legislation in Congress have been unsuccessful.

"This is an emerging trend that is just continuing to increase," Bill McMullan, special agent in charge of the ATF’s Los Angeles office, said during a press conference announcing the July bust. "Criminals are making their own weapons because they’re not able to buy them legally."

Homemade AR-15 type rifles were used in a northern California mass shooting last year and in a rampage at Santa Monica College in 2013, but authorities fear they could do even more damage if they become a staple of the street.

"For the ATF it sometimes makes things a little more challenging when gang members are getting around the laws and trying to find ways to get their hands on guns and creating more violence in their communities," McMullan later told NBC News.

He said he believes that ghost guns are a problem across the nation but that agents are coming across them more in states, like California, "that have stricter gun laws."

Ghost guns are also finding their way to Mexican drug cartels, according to a 2016 report from the Government Accountability Office. "It's international now," said Adam Skaggs, chief counsel for the Giffords Law Center to Prevent Gun Violence, which lobbies for stricter firearms control. "There’s a huge, dangerous underbelly of the gun industry operating online."

Also in July, New Jersey Attorney General Gurbir Grewal sent cease-and-desist letters to online firms that sell "partially-built firearms" in the Garden State, warning that the selling of parts that could make an assault rifle is outlawed there.

"These weapons are illegal in our state," he wrote. "I will bring legal action against your company."