Section 702 of the FISA Act allows agencies like the NSA to eavesdrop on foreign nationals overseas. It prohibits spying on US citizens or anyone who‘s within the United States. (…)

In practice, then, NSA maintains a huge database of information about the communications of US citizens. Agencies like the FBI are allowed to query this database, but only under strict rules. The primary rule is that queries can only be done if they are related to foreign intelligence or are likely to show evidence of a crime.