A US senator revealed Tuesday that he believes a former FBI agent who disappeared in Iran nearly two years ago is being held in a secret prison there. Sen. Bill Nelson shed new light on the disappearance of Robert Levinson during a confirmation hearing for Hillary Rodham Clinton in Washington. Levinson, of Florida, was last seen on Iran's Kish island in March 2007 where he had gone to seek information for a client of his security firm. Nelson said the Iranian government has rebuffed numerous requests for information on Levinson's whereabouts. "The door has been closed at every turn," Nelson said. "We think he is being held by the government of Iran in a secret prison."