WaPo: The Fourth Amendment and querying the 702 database for evidence of crimes by Orin Kerr:
An interesting Fourth Amendment issue has come up in debates over the reauthorization of Section 702 of the Foreign Intelligence Surveillance Act. This is the authority, some will recall, that allows the government to collect from inside the United States the contents of communications of foreigners reasonably believed to be outside the United States without obtaining a warrant. Here’s the question, as I understand it: If a foreigner (a non-United-States person, in the statute) is communicating with a U.S. citizen inside the United States, and the government has collected the communications between them under Section 702 by targeting the foreigner, what are the Fourth Amendment limits on querying the database to find evidence of criminal activity committed implicating the United States person? In other words, after the government has collected the contents under the national security authorities based on the non-U.S. person, what are the limits on switching over to a criminal query that targets the U.S. person?