Also on Tuesday, the Defense Intelligence Agency investigated a case of possible biological agents being concealed in mail sent to the agency at its Bolling Air Force Base Headquarters in Washington, D.

WSJ: Ricin Charges Are Dropped

The biggest concentration is at Wright-Patterson Air Force Base near Dayton, headquarters of the Air Force Materiel Command and the Air Force Research Lab.

FORBES: Military Interests May Sway Vote In Ohio, A Key State For Republicans

Rumsfeld was returning from a visit to Central Command headquarters at MacDill Air Force Base in Tampa, Florida, when he commented to reporters.

CNN: Taliban say Omar safe, deny strike hit leadership compound

Headquarters: MacDill Air Force Base, Tampa, Florida.

CNN: Specials

President Bush, in Florida when the attacks occurred, canceled plans to return to Washington and addressed the nation from Barksdale Air Force Base in Louisiana and later traveled to the U.S. Strategic Air Command headquarters in Nebraska.

CNN: Nation's capital under state of emergency