This data-mining indicated that it might be valuable to do more targeted searches of particular communications “pipelines” (John Yoo’s phrase), looking for more specific information. But that’s where FISA came in. In order to target a particular U.S. person, or to wiretap a particular “facility,” FISA requires that the NSA demonstrate to the FISA court probable cause to believe (i) that the target of the electronic surveillance is a foreign power or an agent of a foreign power, and (ii) that each of the facilities or places at which the electronic surveillance is directed is being used, or is about to be used, by a foreign power or an agent of a foreign power. 50 U.S.C. 1805(a)(3).
Perhaps, as John Yoo suggests in his book, FISA would have prohibited following up on the leads revealed by the data mining with more targeted wiretaps of suspicious “channels” or “pipelines,” “because we would have no specific al Qaeda suspects, and thus no probable cause.”