Henry Hanks links to this New York Times article that suggests the planning for the 9/11 attacks may have originally included five planes, with the fifth plane targeting the White House.
Ramzi Muhammad Abdullah bin al-Shibh, who is now in custody in Pakistan, applied to enter the United States to obtain training at a Florida flight school but was denied entry into this country. While living i n Germany, bin al-Shibh shared an apartment with Mohammed Atta.
According to The Times,
But investigators said suspicions were growing that Mr. bin al-Shibh may have intended to lead a fifth hijacking group. Their belief is based on other information, including interviews of other Qaeda detainees and Mr. Lindh, the American who was sentenced on Friday to 20 years in prison for fighting with the Taliban in Afghanistan and whose credibility is still being weighed. Mr. Lindh told the authorities that he had heard that five attacks were planned.
. . .
In addition, investigators have examined more closely Mr. bin al-Shibh’s unsuccessful efforts to obtain a visa to enter the United States, where he had signed up for flying lessons at a Florida aviation academy. In August 2000, he paid $2,200 as a deposit for flight training, an amount officials said was enough to convince them that Mr. al-Shibh seriously intended to learn to fly.
Source:
9/11 Inquiry Eyes Possible 5th Pilot. David Johnsont on and Don Van Natta Jr. The New York Times, October 11, 2002.