According to the USA Patriot Act, librarians face criminal penalties if they inform their patrons that they are under investigation by the FBI.
While this prior restraint on free speech is categorically unconstitutional, (as are many other aspects of the act), the courts have not yet recognized this fact, thus making it risky for well-meaning librarians to do the right thing w/out risking their own freedoms.
What to do?
An enterprising, free-thinking, occasional librarian, Jessamyn West, has come up with a pretty ingenious solution. Here’s an example:
See the rest of her technically legal signs at librarian.net. Thanks to Boing Boing and The Shifted Librarian for this gem.