FindLaw: U.S. Constitution: Fourth Amendment

The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

USNews.com: The White House says spying on terrorism suspects without court approval is OK. What about physical searches? By Chitra Ragavan

In December, the New York Times disclosed the NSA’s warrantless electronic surveillance program, resulting in an angry reaction from President Bush. It has not previously been disclosed, however, that administration lawyers had cited the same legal authority to justify warrantless physical searches. But in a little-noticed white paper submitted by Attorney General Alberto Gonzales to Congress on January 19 justifying the legality of the NSA eavesdropping, Justice Department lawyers made a tacit case that President Bush also has the inherent authority to order such physical searches. …

During Senate testimony about the NSA surveillance program, however, Gonzales was at pains to avoid answering questions about any warrantless physical surveillance activity that may have been authorized by the Justice Department. …

In 1975 and 1976, an investigative committee led by then Sen. Frank Church documented how the FBI engaged in broad surveillance of private citizens and members of antiwar and civil rights groups, as well as Martin Luther King Jr. The committee’s hearings and the executive-branch abuses that were documented in the Watergate investigation led to numerous reforms, including passage of the Foreign Intelligence Surveillance Act in 1978.