The tweet comes one day before Cody Wilson, a self-proclaimed "crypto-anarchist" and gun rights advocate, is to make the open-source digital files for making so-called "ghost guns" available for download.

Politicians, gun control advocates, and members of law enforcement expressed concerns that anyone -- from a teenager to a convicted felon -- could make untraceable weapons, including plastic ones that could evade metal detectors.

The technology presents Trump with tough questions about protecting the public, the limits of gun ownership rights and his own political fortunes.

Amid last-minute legal maneuvers to stop Wilson, Trump weighed in on the debate, revealing that he had spoken to America's main pro-gun lobby, the National Rifle Association, about the topic.

"I am looking into 3-D Plastic Guns being sold to the public," Trump said, in apparent skepticism about their use. "Already spoke to NRA, doesn't seem to make much sense!"

- Legal challenge -

In June, after a five-year legal battle between Wilson and the federal government, Trump's administration granted him permission to operate his website Defcad, envisioned as the WikiLeaks of firearms.

Wilson, who has called gun control "a fantasy," told The Washington Post that the government's efforts to stop him from publishing the computer code needed to build 3D-printed guns was akin to stifling free speech.

"[Code] is the essence of expression," he told the newspaper. "It meets all the requirements of speech -- it's artistic and political, you can manipulate it, and it needs human involvement to become other things."

But eight states filed a federal lawsuit Monday in a last-minute effort to stop Wilson, saying the issue is one of public safety.

Karl Racine, the attorney general for the District of Columbia, said Tuesday that allowing Wilson to proceed would be "reckless and would create chaos and violence in the streets of the United States."

During his 18-month presidency, which has not been spared from mass shootings in schools and elsewhere, Trump has occasionally seemed to favor tougher gun regulations, only to later buckle under pressure from his base and donors.

Democrat Nancy Pelosi, the minority leader in the US House of Representatives, condemned the administration's settlement that will allow Wilson's website to go live on Wednesday, barring further court action.

"This decision is a death warrant for countless innocent men, women and children," Pelosi said in a statement. "For the sake of all our safety and lives, it must be reversed immediately."

- Global problem -

While Wilson has somehow become the public face of homemade weapons technology, the phenomenon is bigger than his website alone.

Earlier in July, Los Angeles police showcased an arsenal of so-called "ghost guns" seized from gang members during a six-month undercover operation.

The weapons, including AR-15-style semiautomatic rifles, were fashioned from kits purchased online, according to police. Wilson's website will also feature blueprints for AR-15-style arms.

Wilson wants to supercharge the "ghost gun" trend by building the definitive repository of information, where even a novice can download digital code and build a gun with a 3D printer.

Jonathan Lowy of the Brady Center to Prevent Gun Violence told AFP that the implications could be even greater for other countries.

"It's certainly a huge international problem, particularly given that many other countries have much stronger gun laws than in America," Lowy said.

"So, in those countries, there are many people who shouldn't have guns and could not get them unless they can get their hands on a 3D-printed gun."

Wilson is unfazed by such arguments, countering that the US Constitution's Second Amendment, which guarantees the right to bear arms, should extend to a person's right to make their own guns uncontrolled by authorities.

In the five years since his legal battle began, his company Defense Distributed has grown to 15 employees inside a non-descript warehouse in the Texas state capital of Austin.

They have created a plastic 3D-printable handgun called the "Liberator" and amassed digital design files for a number of other weapons.