U.S. government works are not protected under U.S. copyright law.  This does not mean that all U.S. government works are publicly available without restriction.  However, the government does not restrict the use or distribution of most categories of U.S. government works.  Certain types of official works of government bodies, such as statutes and court decisions, are generally not copyrighted.  For the reasons like national security, U.S. government works undergo appropriate security reviews by the releasing agency before placing it for public availability

U.S. government may retain copyright ownership in a copyrighted work transferred to it by way of assignment. In that case, such work retains its copyright protection.

A work of the United States government is a work prepared by an officer or employee of the United States government as part of his or her official duties.  However, copyright law does not consider certain categories of people like contractors and grantees who work with the government as U.S. government employees.  The works of the state and local government is not a U.S. government work.