Blueprints for 3D-printed guns, published online four days early amid last-minute attempts to ban them, have been downloaded thousands of times.
"A gun that can evade the detection system, it just defies common sense, and yet this is what the Trump administration has done", said Nelson.
Defence Distributed agreed to temporarily block Pennsylvania residents from downloading the plans after state officials went to Federal Court in Philadelphia on Sunday in search of an emergency order.
U.S. District Court Judge Robert Lasnik granted a temporary restraining order on Tuesday night barring a trove of downloadable information about creating the do-it-yourself weapons. State Department officials said the plans violated USA export laws.
On issue after issue, the Trump administration's M.O. when there's a crisis is to say, 'We'll look into it.
"All you need is a little money and you can download a blueprint from the internet to make a gun at home", said Senate Minority Leader Chuck Schumer.
Gun access advocacy group Defense Distributed was due to put downloadable gun blueprints online on Wednesday.
But just how unsafe are so-called 3-D printed guns, and why has this emerged as a hot topic?More news: Leaked Photos of the Google Pixel 3 XL?
More news: Delegation joins calls for action after Trump-Putin meeting
More news: Putin: Your toolkit to help understand the story
Washington's Attorney General, Bob Ferguson, said at a press conference before filing the lawsuit on Monday that the Trump administration's settlement was "unprecedented and not only disastrous for public safety but undermines our state laws meant to keep firearms out of the hands of risky individuals". He says 3D-printed guns present a real and present danger because they're both unregulated and untraceable.
Lawmakers say that the 3D printable guns are unsafe because they are unregistered and can be undetectable by metal detectors because they are made from plastic.
The president expressed doubt, saying the move "doesn't seem to make much sense!".
These basic guns can be made by anyone who owns a 3D printer, which uses plastic or other materials to build up an object layer by layer.
The issue arose in 2013 when Texas-based Defense Distributed published 3D printing plans online, resulting in about 100,000 downloads.
Defense Distributed's files include 3-D printable blueprints for components that would go into the making of a version of the AR-15 semi-automatic assault rifle, a weapon that has been used in many U.S. mass shootings. Because it's not necessarily illegal in the USA, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives, to produce your own guns.
It was downloaded about 100,000 times until the USA 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.