• it is a photograph, a cinematographic work or a sound recording and at least 50 years have passed since the end of the year of its publication (or since the year of creation for photographs from before independence in 1947, as per the Copyright Act 1911).
• it is a governmental or anonymous work and at least 50 years have passed since the end of the year of its publication.
  

• its author died before 1954;
• the death date of its author is not known, and it was created before 1904;
• it is an anonymous work, a pseudonymous work, or a work made for hire, and it was created before 1904.

The above provisions are contained in 17 U.S.C. § 303. See also this page for more information.