Police say the suspect had a ghost gun, possibly made with a 3D printer, when he was arrested on Monday. Here's what to know about the increasingly widespread weapons and efforts to regulate them.

The ATF classifies the kits as firearms under the 1968 Gun Control Act, but kit manufacturers and sellers challenged the rule in court, asserting that the ATF had exceeded its authority.

The order voids the lower court orders and allows ATF regulations to go into effect pending further litigation.