It's called a workable alibi.

U.S. investigators have dismissed Czech accounts of an April 2001 meeting in Prague between suicide hijacker Mohammed Atta and al-Ani ...

[ ... ]

... U.S. investigators said that Atta was in the United States during the time he was supposed to have been meeting with al-Ani.

At the moment, the U.S. supplies the alibi for dead Atta, and without Atta around to muck up the works, it will likely stand.

Then again, Atta might never have been to Prague.

Recall, however, that, while alive, Atta was not under close surveillance -- hence the partial success of the attacks on Sept 11.

2000
June 2
According to Czech officials, Mohamed Atta arrives in Prague by bus and stays there for 24 hours.

June 3
Atta arrives in Newark, New Jersey aboard a Czech Airlines flight from Prague with a six month tourist visa for the US. He has shaved off his beard, has begun to wear western style clothing again and appears to have a large amount of money at his disposal.
(Source)

Still interesting,
Dan