[swarthyguy]

People also forget that the 2001 G8 Conference was held in Genoa that summer, and the US insisted on anti aircraft missiles around the luxury cruise ship and other venues in that city.

So the threat was no secret.

And remember that plane that crashed into the WH in 94/96?

And the foiled plot in France when jihadis tried to crash a plane into the Eiffel Tower but were foiled during a fake refueling stop?

[fourhorsemen]

Again I'd note that the attacks of 9/11 were not the first time someone tried to hijack a plane and use it to attack buildings in the United States. Samuel Byck tried to assassinate Richard Nixon via this method in 1974.

See:
http://www.findarticles.com/cf_dls/m1571/23_18/87917222/p1/article.jhtml