Washington ? There may be additional e-mails that could have tipped off law enforcement or military officials to the Fort Hood shooter before he went on his deadly rampage, the chairman of the Senate Armed Services Committee said Friday.

The U.S. government intercepted at least 18 e-mails between Maj. Nidal Malik Hasan and Anwar al-Awlaki, a radical American-born cleric. They were passed along to two Joint Terrorism Task Force cells led by the FBI, but a senior defense official said no one at the Defense Department knew about the messages until after the shootings. The official spoke on condition of anonymity in order to discuss intelligence procedures.

Sen. Carl Levin, D-Mich., said after a briefing from Pentagon and Army officials that his committee will investigate how those and other e-mails involving the alleged shooter, Hasan, were handled and why the U.S. military was not made aware of them before the Nov. 5 shooting.