These works are in the public domain and can be used by anyone for any legal purpose without permission:

• Works created by U.S. government employees in the scope of their employment
• Works for which copyright protection has expired
• Works that do not contain the requisite originality (e.g., facts, blank forms)
• Works that contain a notice from the copyright owner expressly rejecting any claim of copyright and placing the work in the public domain

Unless you are sure a work falls into one of these categories, you must assume the work is protected by copyright.