[Memorandum for the Attorney General: Re: The Foreign Intelligence Surveillance Act and the President's Ability to Engage in Warrantless Searches for Purposes of National Security, November 2, 2001] [open pdf - 74KB]
This November 2, 2001 memorandum from John C. Yoo, the Deputy Assistant Attorney General of the Office of Legal Counsel, is addressed to the Attorney General. The contents of the document have been redacted but what is available concerns the Foreign Intelligence Surveillance Act (FISA) and "the President's ability to engage in warrantless searches that protect the national security."
U.S. Dept. of Justice: http://www.justice.gov/