This week has brought fresh questions about what U.S. officials knew about Sept. 11 hijacker Mohammed Atta -- and when they knew it.

The questions concern a secret military unit called "Able Danger." According to Curt Weldon, a Republican congressman who has sought to publicize the matter, Able Danger was tracking Atta and three other hijackers more than a year before the attacks.

Weldon alleges the military team tried to share its intelligence with the FBI, but government lawyers warned it not to.

The 9/11 Commission will soon release details on its inquiry into the Able Danger unit and what the group knew about Atta before he took part in the attacks of Sept. 11, 2001.

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