Review of standards of probable cause for obtaining a FISA warrant
from
https://epic.org/privacy/surveillance/fisa/For U.S. persons, the FISC judge must find probable cause that one of four conditions has been met:
(1) the target knowingly engages in clandestine intelligence activities on behalf of a foreign power which "may involve" a criminal law violation;
(2) the target knowingly engages in other secret intelligence activities on behalf of a foreign power under the direction of an intelligence network and his activities involve or are about to involve criminal violations;
(3) the target knowingly engages in sabotage or international terrorism or is preparing for such activities; or
(4) the target knowingly aids or abets another who acts in one of the above ways.
An order of the FISC may approve electronic surveillance of an agent of a foreign power for ninety days and of a foreign power for a year. Extensions may be granted on the same terms.