A September 25, 2001 memorandum released March 2, 2009 by the Justice Departent asserts in sweeping language that the attacks of September 11, 2001 justified warrantless surveillance in the U.S. as an act of collective self-defense. "If the government's heightened interest in self-defense justifies the use of deadly force, then it certainly would also justify warrantless searches," the memo says.
The memo, authored by John Yoo, then the Deputy Assistant Attorney General, argues that warrantless national security searches that were unreasonable under the Fourth Amendment prior to the September 11 attacks may well be reasonable after those attacks because the government's interest in conducting the search had strengthened. Thus, though the text of the Fourth Amendment hadn't changed, broader warrantless search authority resulted from the attacks, the memo argues.
However, the memo also suggests that some intelligence surveillance authorized by the USA PATRIOT Act may be unconstitutional. Prior to the PATRIOT Act, intelligence surveillance could be conducted under the Foreign Intelligence Surveillance Act only if the "primary purpose" of the surveillance was to gather foreign intelligence. Relaxed probable cause standards applied. The government had to prove probable cause that the target of surveillance was an agent of a foreign power - such as a foreign terrorist organization or foreign government - instead of full probable cause of crime.
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