Two former officials familiar with the data-sifting efforts said they work by starting with some sort of lead, like a phone number or Internet address. In partnership with the FBI, the systems then can track all domestic and foreign transactions of people associated with that item -- and then the people who associated with them, and so on, casting a gradually wider net. An intelligence official described more of a rapid-response effect: If a person suspected of terrorist connections is believed to be in a U.S. city -- for instance, Detroit, a community with a high concentration of Muslim Americans -- the government's spy systems may be directed to collect and analyze all electronic communications into and out of the city.
The haul can include records of phone calls, email headers and destinations, data on financial transactions and records of Internet browsing. The system also would collect information about other people, including those in the U.S., who communicated with people in Detroit.
[...] The NSA, which President Truman created in 1952 through a classified presidential order to be America's ears abroad, has for decades been the country's largest and most secretive intelligence agency. The order confined NSA spying to "foreign governments," and during the Cold War the NSA developed a reputation as the world's premier code-breaking operation. But in the 1970s, the NSA and other intelligence agencies were found to be using their spy tools to monitor Americans for political purposes. That led to the original FISA legislation in 1978, which included an explicit ban on the NSA eavesdropping in the U.S. without a warrant.