Warrantless surveillance of Americans is legal under the 1978 Foreign Intelligence Surveillance Act
FISA requires a warrant for wiretaps on Americans
FISA lets the federal government eavesdrop on overseas communications without a warrant, but not Americans in the United States. The Associated Press reported in a December 12, 2012, article:
[T]he law allows the government to monitor overseas phone calls and emails without obtaining a court order for each intercept.
The law does not apply to Americans. When Americans are targeted for surveillance, the government must get a warrant from a special 11-judge court of U.S. district judges appointed by the Supreme Court.
The New York Times reported in 2005 that President Bush authorized the National Security Agency to wiretap Americans and others inside the U.S. without a warrant.