"Unless otherwise noted, information presented on CMH Online is considered public information and may be distributed or copied for non-commerical purposes. Use of appropriate byline/photo/image credits is requested. If copyrighted or permission restricted materials are posted on CMH Online, the appropriate credit is given. Visitors wishing to repost or use such materials for their own projects should make separate arrangements for permission with the owner."In other words, the site claims to sift out those works that are under copyright by labeling them with a credit. Everything unlabeled presumably is in the public domain. Assuming you can trust the site's filtering of material, then, as the Supreme Court has stated, you can do anything you want with those materials, with or without attribution to the author.
By the way, although works prepared by federal government employees are in the public domain, you may be surprised to learn that the U.S. government -- though it rarely exercises the right -- is legally entitled to claim copyright outside the U.S. (see page 59 of link).
P.S. The licensing dilemma. Finally -- and this doesn't seem to be an issue at the CHM site -- we're always concerned about the trend to license public domain material. For example, if you check out the terms for this Department of Defense site, you'll see that some restrictions have been placed on use of U.S. government imagery. That claim is made on the basis of a license -- that is, you agree that as a condition of using the website, you will abide by the rules regarding photo use. Generally such licenses are only effective if the user must click to accept or demonstrate some action taken to indicate assent.