Attorney General Jeff Sessions said the Justice Department will "use every available tool" to prosecute people who make or possess undetectable plastic firearms.
The Undetectable Firearms Modernization Act, which has been around in some form for decades, prohibits firearms that don't set off walk-through metal detectors or other major security screening devices.
The Department of Justice issued a statement by Sessions on Thursday saying it would "take action to ensure that individuals who violate the law by making plastic firearms and rendering them undetectable, will be prosecuted to the fullest extent."
But his statement comes after the DOJ filed documents aiming to allow the publication of blueprints for 3D-printed guns, which critics say are largely untraceable.
Last month, a federal judge temporarily blocked the online publication of blueprints after several states and the District of Columbia sued. They argued it's hard for metal detectors to spot the firearms and that web blueprints will make it easier for people who aren't allowed own guns to get their hands on them.