Once the applicable copyright term has expired, the work passes into the “public domain” and may be freely used by anyone for any purpose.
Any work that was first published in US before 1923 is now in the public domain under US law.
“Publication” means “the distribution of copies or phonorecords of a work to the public by sale or other transfer of ownership, or by rental, lease, or lending. The offering to distribute copies or phonorecords to a group of person for purposes of further distribution, public performance, or public display constitutes publication. A public performance or display of a work does not of itself constitute publication.” (17 USC section 101)
Works created by federal officers or employees as part of their official duties such as presidential speeches, congressional reports, and federal judicial opinions are in the public domain.