A judge has blocked the publication of blueprints for parts of a gun that can be made by a 3D printer and, it is claimed, would be undetectable.
The Trump administration had previously given the go-ahead for the online publication of the plans, which show how to make a firearm from printable parts.
The gun's components can be made with a printer that uses plastic instead of ink. Layers of plastic are built up until they make three-dimensional plastic pieces.
The company behind the plans, Defense Distributed, reached an agreement with the federal government in June allowing it to make the plans for the guns available for download, starting this week.
It would have allowed anyone with access to a printer to potentially build a lethal weapon, which opponents say cannot be seen by a metal detector or tracked to a licence holder.
But US District Judge Robert Lasnik has now issued an order that prevents that, for now.
The right to access the blueprints was originally forbidden by the State Department after the printable gun was invented in 2013 by a gun-rights activist Cody Wilson.
A four-year legal battle followed between digital weapons publisher Defense Distributed, pro-gun activists the Second Amendment Foundation and the federal government, which concluded with a surprise decision that US citizens can "access… use and reproduce" the plans.
It has been reported that, although the blueprints were supposed to be made available from Wednesday, they were released early and have already been downloaded thousands of times.
At a news conference on Tuesday, Connecticut Senator Richard Blumenthal said that if President Trump did not block sale, blood was "going to be on his hands".
The US president appeared to agree with the concerns when he tweeted he was "looking into" the issue of the guns and had spoken to the National Rifle Association about it.
He added that it didn't "seem to make much sense!".
Other Republicans have also expressed concern, with Alaska Senator Lisa Murkowski tweeting: "Even as a strong supporter of the Second Amendment – this is not right."
Washington state attorney general Bob Ferguson had announced on Monday that he would be suing the State Department to get it to change its mind on the so-called "ghost guns".
In a statement, Mr Ferguson said: "These downloadable guns are unregistered and very difficult to detect, even with metal detectors, and will be available to anyone regardless of age, mental health or criminal history."
The NRA said in a statement that "anti-gun politicians" and some members of the news media had been wrongly claiming 3D printing technology would "allow for the production and widespread proliferation of undetectable plastic firearms."
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A 1988 federal law – drafted with NRA support – barred the manufacture, sale or possession of an undetectable firearm.
But Eric Soskin, a lawyer for the US Justice Department, told Tuesday's federal hearing in Seattle, the government had reached a settlement with Defense Distributed because the regulations were designed to restrict weapons that could be used in war, and the online guns were no different from the weapons that could be bought in a store.