The detention of California peace activist Ali Shakeri was widely suspected, as unsourced reports had suggested he was being held over possible accusations of security-related wrongdoing.
Shareki's detention was announced today by Iranian Foreign Ministry spokesman Mohammad Ali Hosseini.
Two U.S.-Iranian scholars -- Haleh Esfandiari and Kian Tajbakhsh -- are also in jail, accused of spying.
A fourth U.S.-Iranian citizen, Radio Farda reporter Parnaz Azima, has been charged with "antirevolutionary" activities and is free on bail.
The U.S. State Department calls the charges "groundless."