The Biden administration took action Tuesday to close a “ghost gun” loophole following an August rule issued by the Bureau of Alcohol, Tobacco, Firearms and Explosives.

ATF sent an open letter to “all federal firearms licensees” outlining how a number of “partially complete, disassembled, or nonfunctional” gun kits are now classified as “readily” available weapons, requiring each to carry a serial number and for dealers to run background checks prior to sales.