...To monitor the communications of a "U.S. person" (a citizen, legal permanent resident, or U.S. corporation) under the Foreign Intelligence Surveillance Act (FISA), the FBI needs probable cause to believe the target is an agent of a foreign power. In Page's case, the relevant definition of a foreign agent would be someone who "knowingly engages in clandestine intelligence gathering activities for or on behalf of a foreign power," or who knowingly assists such activities, when they "involve or may involve" violating a criminal statute. When a U.S. person is the target, a warrant cannot be issued "solely upon the basis of activities protected by the first amendment."

Unlike when it seeks a warrant in an ordinary criminal case, the FBI did not need to show probable cause to believe that Page had broken the law, only that he was an agent of a foreign power, which might (or might not) involve illegal espionage. The New York Times says the FBI had to show Page was "probably an agent of a foreign power." But if probably means "more likely than not" (i.e., supported by "a preponderance of the evidence"), that is incorrect. Probable cause, which the Supreme Court has described as "a fair probability," is supposed to be stronger than a hunch or a reasonable suspicion, but it is weaker than a preponderance of the evidence, the standard that applies to verdicts in civil cases. To put it another way, the FBI could get a warrant for an American who was probably not a foreign agent....