A federal judge in Seattle has issued a temporary restraining order to stop the release of blueprints to make untraceable and undetectable 3D-printed plastic guns.
Eight Democratic attorneys general filed a lawsuit on Monday seeking to block the federal government’s settlement with the company that makes the plans available online.
They also sought a restraining order, arguing the 3D guns would be a safety risk.
US District Judge Robert Lasnik issued the order on Tuesday afternoon.
The company behind the plans, Texas-based Defence Distributed, had reached a settlement with the federal government in June that allows it to make the plans for the guns available for download on Wednesday.
The restraining order puts that plan on hold for now.
In the meantime, Congressional Democrats have urged President Donald Trump to reverse the decision to let Defense Distributed publish the plans.
Mr Trump said he is consulting with the National Rifle Association (NRA) over whether it makes sense for the Austin company to publish the blueprints.
Mr Trump tweeted he is “looking into 3-D Plastic Guns being sold to the public,” adding he “already spoke to NRA”.
The president expressed doubt, saying the move “doesn’t seem to make much sense!”
Joining the Seattle suit were Democratic attorneys general in Massachusetts, Connecticut, New Jersey, Pennsylvania, Oregon, Maryland, New York and the District of Columbia.
Separately, attorneys general in 21 states urged US secretary of state Mike Pompeo and attorney general Jeff Sessions to withdraw from the settlement with Defense Distributed, saying it "creates an imminent risk to public safety".
Disintegrate
People can use the blueprints to manufacture a plastic gun using a 3D printer. But gun industry experts have expressed doubt that criminals would go to the trouble, since the printers needed to make the guns are very expensive, the guns themselves tend to disintegrate quickly and traditional firearms are easy to come by.
Unlike traditional firearms that can fire thousands of rounds in their lifetime, experts say the 3D-printed guns normally only last a few rounds before they fall apart.
They do not have magazines that allow the usual nine or 15 rounds to be carried; instead, they usually hold a bullet or two and then must be manually loaded afterwards. They are not usually very accurate.
Cody Wilson, the founder of Defense Distributed, first published downloadable designs for a 3D-printed firearm in 2013.
It was downloaded about 100,000 times until the US state department ordered him to cease, contending it violated federal export laws since some of the blueprints were downloaded by people outside the United States.
The state department reversed course in late June, agreeing to allow Wilson to resume posting the blueprints. The files were published on Friday.
The company filed its own suit in Texas on Sunday, asserting that it is the victim of an “ideologically fuelled programme of intimidation and harassment” that violates the company’s First Amendment rights.
Meanwhile, Defense Distributed agreed to temporarily block Pennsylvania residents from downloading the plans after state officials went to federal court in Philadelphia on Sunday seeking an emergency order.
The company said it has also blocked access to users in New Jersey and Los Angeles.