The Supreme Court upheld federal restrictions on the kit guns in March, but gun rights groups hope the Trump administration -- or the courts -- will still roll them back.

The court says ghost guns, which are virtually untraceable weapons that require assembly, are considered "firearms" and subject to ATF regulation.

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.