My immediate reaction was that Atta wrote those letters prior to 9/11 (to be mailed by someone else later) and left a clue so the world would know he sent them.
Remember, he sought treatment for very red hands that the pharmacist thought looked like a chemical burn. Strong Clorox solution?
I have a thing about coincidences. On this map is Greendale, Mitchell Airport, Rainbow Commercial Airport, behind Mitchell airport is Pennsylvania Avenue, and not far from there is Oklahoma Avenue. There is also a university and a large Muslim population. A great place to get lost.