"Fisa court-approved policies allow the NSA to retain and make use of "inadvertently acquired" domestic communications if they contain usable intelligence, information on criminal activity, threat of harm to people or property, are encrypted, or are believed to contain any information relevant to cybersecurity..."
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Revealed: the top secret rules that allow NSA to use US data without a warrant
www.guardian.co.uk
Fisa court submissions show broad scope of procedures governing NSA's surveillance of Americans' communication
http://www.guardian.co.uk/world/2013/jun/20/fisa-court-nsa-without-warrant