Works that are prepared by employees of the government pursuant to their official duties are in the public domain due to an express statutory provision prohibiting the U.S. Government from claiming copyright in such works. ARL has published an issue brief explaining the statutory provision prohibiting copyright protection for works of the U.S. Government, which is available here.
The brief details the statute and also points to a number of U.S. Government agency websites and White House website statements affirming that works by the U.S. Government are not eligible for copyright protection. The Department of Labor, the Patent and Trademark Office, the Department of Energy, the National Science Foundation, the Department of Agriculture and the Department of Treasury are just some examples of agencies that include express recognition that materials produced by its government employees are not eligible for copyright protection. The White House website similarly notes in its Copyright Policy, “Pursuant to federal law, government-produced materials appearing on this site are not copyright protected.”