[surferbayarea]

Seems it does include US citizens.

Per "https://en.wikipedia.org/wiki/Foreign_Intelligence_Surveilla... : "and "agents of foreign powers" (which may include American citizens and permanent residents suspected of espionage or terrorism)."

[lern_too_spel]

You can look at the definition here: https://www.law.cornell.edu/uscode/text/50/1801. Suspicion of espionage or terrorism isn't enough to be considered an agent of a foreign power. A US citizen must be known to have aided a foreign power in espionage or sabotage and done so knowingly.