TEL AVIV — Defense Secretary Leon E. Panetta said Monday that U.S. officials have been trying to broker the release of an alleged Israeli spy held by Egypt, and he raised hopes that he could win the prisoner’s release during a visit to Cairo this week. Ilan Grapel, 27, a dual U.S.-Israeli citizen and a law student at Emory University in Atlanta, was arrested in June in Cairo on suspicion of espionage.