(CNN) -- As many as nine Americans may have been aboard the Gaza relief flotilla attacked by the Israeli military, and are now being held by Israeli authorities, a State Department official said Monday.

The official, who had no details about their identities, conditions or whereabouts, said U.S. consular officials were expecting to interview the Americans on Tuesday.

Separately, Jonathan Slevin, a spokesman for the activist group Free Palestine Movement, said a group of five Americans participated in the flotilla, including former U.S. Ambassador Edward Peck.

Slevin said the Americans boarded a ship called the Sfendoni in Athens, Greece. He said there had been no communication with the Americans since the flotilla was intercepted by the Israelis.