I posted all this on another board:
But the idea of planes flying into things to blow them up should have been obvious to anybody charged with counter terrorism planning - - the Japanese flew an awful lot of planes into military targets during World War II.
In 1974, a psychotic celebrity stalker named Samuel Byck attempted to storm a parked aircraft and hijack it, with the intent of crashing it into the White House in order to kill Richard M. Nixon. The police, who spend an awful lot of time imagining scary things like "what if that psycho over there actually manages to steal that plane" shot him, which put an end to his plan to - - again - - fly a plane into the White House to kill the President. (FYI, Byck then committed suicide rather than surrender to the police.)
More from Rotten.com on other reasons why 9/11 wasn't really "unthinkable":
http://www.rotten.com/library/crime/terrorism/terror-tactics/kamikaze-jet-hijacking/The earliest documented warning of what lay ahead for the United States currently appears to have come in late 1992 and early 1993, when informants working for the Egyptian and U.S. governments reported that a terrorist who had infiltrated the Sudanese Air Force was planning to bomb an Egyptian presidential mansion then crash his fighter into the U.S. Embassy in Cairo.
The terror plot, which was linked to al Qaeda, clearly showed exactly how airplanes appealed to terrorists. But there was so much worse to come.
By the time of the above incident, people were beginning to wake up the possibility that someone could pull off a suicide hijacking, and the Pentagon commissioned a study to look into the risk in 1993. In early 1994, a disgruntled FedEx worker tried unsuccessfully to storm a jet's cockpit and crash the plane into a company facility. It didn't take long for the concept to advance dramatically.
The date was September 11, 1994. YES, THAT'S RIGHT, SEPTEMBER 11. After downing a lot of crack cocaine and booze, Frank Eugene Corder climbed into an airplane and took off. Had he spent a little less time partying, the crash would have come that evening. Instead, around 2 a.m. the morning of Sept. 12, Corder attempted to crash the stolen airplane into the White House. He missed by a matter of yards.
Like Timothy McVeigh, Corder was a Army veteran. Like Jose Padilla, he had passed a few stints in prison. Like Mohammed Atta and Zacarias Moussaoui, he was a student pilot. He was officially designated a lone, uh, planeman and quickly forgotten.
(snip)
On December 24, 1994, Algerian Armed Islamic Group hijacked an AirFrance commercial jet and threatened to crash it into the Eiffel Tower. AIG is an Islamic terrorist group tied to Osama bin Laden and al Qaeda. As warnings go, this was a dead giveaway.
The plot had been scheduled for New Year's Eve, but the operatives carrying it out had moved it up to Christmas Eve due to concerns about being caught. The change of plans ended up putting the flight on a refueling stop before it could hit the tower, and French commandos stormed the plane, successfully killing the hijackers.