[Mitchell]

Gross said "a certain person" also tried to enter the Czech Republic without a valid visa in May 2000, but there was no hard evidence this person was in fact Atta.

This is evidence that he was traveling under a false identity at that time. Of course there are no airline or visa records.

[Mitchell]

I just realized that that was a different trip (in 2000, not 2001). But it still shows that Atta would sometimes travel using false documents (or otherwise avoid having records made of his travels).