What We Know, Atta Minimum

Like the assassination of JFK , 9/11 seems destined to be a developing story in perpetuity. As such, Mohamed Atta, the ringleader of the Sept. 11 terrorist attacks, is still finding new ways to haunt our national imagination nearly four years after his death. According to various news accounts:

A. There were two Attas in Prague in summer 2000 (who may or may not have met there with Iraqi intelligence).
B. There were three Attas (two of whom who may or may not have in Prague in summer 2000 and met there with Iraqi intelligence, and two of whom were terrorists, but not the same ones).
C. The U.S. government's Able Danger program had Atta under surveillance prior to 9/11.
D. The U.S. government's Able Danger program did not have Atta under surveillance prior to 9/11.

My question: which of the choice(s) above is/are most plausible? (They're not necessarily mutually exclusive.)

More here courtesy Slate and Hit & Run.