The Gun Control Act of 1968 (also known as GCA or GCA68, and codified as Chapter 44 of Title 18, United States Code) is a federal law in the United States that broadly regulates the firearms industry and firearms owners.

...

Prohibited Persons
The original GCA prohibits firearms purchase and ownership by certain broad categories of individuals thought to pose a threat to public safety. However, this list was in contradiction between the House and the Senate versions of the bill, and led to great confusion. This list was later augmented, modified, and clarified in the Firearms Owners' Protection Act of 1986. The 1986 list is:

Anyone who has been convicted in any court of, a crime punishable by imprisonment for a term exceeding 1 year.
Anyone who is a fugitive from justice.
Anyone who is an unlawful user of or addicted to any controlled substance.
Anyone who has been adjudicated as a mental defective or has been committed to a mental institution.
Any alien illegally or unlawfully in the United states or an alien admitted to the United states under a nonimmigrant visa.
Anyone who has been discharged from the US Armed Forces under dishonorable conditions.
Anyone who, having been a citizen of the United states, has renounced his or her citizenship.
Anyone that is subject to a court order that restrains the person from harassing, stalking, or threatening an intimate partner or child of such intimate partner.
Anyone who has been convicted of a misdemeanor crime of domestic violence. (See the Lautenberg Amendment.)
A person who is under indictment or information for a crime punishable by imprisonment for a term exceeding one year cannot lawfully receive a firearm. Such person may continue to lawfully possess firearms obtained prior to the indictment or information.

The Brady Handgun Violence Prevention Act of 1993 created a national background check system to prevent firearms sales to such "prohibited persons."