Wiretapping, FISA, and the NSA
I always feel like somebody's watching me..
The Foreign Intelligence Surveillance Act (FISA)
Nevertheless, the executive branch has another mechanism for obtaining court orders to intercept communications (including email) if the government doesn't believe that it has evidence of a crime. The Foreign Intelligence Surveillance Act allows the government to get an interception or seizure order (or a secret search warrant) by proving to a special super-secret court that the purpose of the surveillance is to obtain foreign intelligence, including (as amended by the USA Patriot Act) intelligence about terrorism.
FISA orders are directed at interceptions of "US Persons", meaning US citizens or permanent resident aliens, or US corporations. Thus, if a US person is the target of the surveillance, FISA, by its terms, applies. If the US person is not the target, but is otherwise intercepted, the surveillance is OK as long as there are appropriate minimizations procedures in place.
Prior to the enactment of FISA, domestic wiretaps were routinely done for "national security purposes" under nothing more than Presidential authority. Presidents from Roosevelt to Nixon ordered domestic wiretaps to protect national security. Indeed, prior to the enactment of the FISA statute, there used to be an exception in the wiretap criminal statute that provided, "[n]othing contained in this [statute] shall limit the constitutional power of the President to take such measures as he deems necessary to protect the Nation against actual or potential attack or other hostile acts of a foreign power, to obtain foreign intelligence information deemed essential to the security of the United States, or to protect national security information against foreign intelligence activities..." The Nixon administration used this exception to conduct surveillance and interception without warrants on a host of domestic "subversive" groups. When this was revealed, Congress stepped in to limit the abuses by giving the President a mechanism for conducting foreign intelligence (and now terrorism) investigations by passing the Foreign Intelligence Surveillance Act (FISA).
FISA and Presidential power
With the enactment of the FISA statute, this provision was changed to essentially read that FISA now "shall be the exclusive means by which electronic surveillance and the interception of domestic wire, oral, and electronic communications may be conducted." Thirty-three years ago, the US Government tried to rely on pure Presidential power to engage in domestic surveillance of domestic subversive groups without a warrant. US Supreme Court rejected the government's contention that the courts were not prepared to deal with the sensitive classified information, could not make informed decisions about the threats to national security, and that the President had independent authority to order these wiretaps without the Courts. Even if the wiretaps were "reasonable" the Supreme court opined, they violated the Fourth Amendment. Shortly thereafter, the same court found that even the Attorney General could be held liable for authorizing these "national security" wiretaps in that case against a group planning to bomb bridges and tunnels. It was this precedent - establishing that a government official's immunity for ordering such illegal wiretaps is only limited - that Supreme Court nominee Samuel Alito sought to reverse when he was advising the Reagan Administration's Justice Department.