Larry Silverstein, who bought the WTC towers only a few months
before the attack, was a close friend of Ariel Sharon and Benjamin
Netanyahu, the most powerful men in the Israeli government. He
insisted on a clause in the insurance policy that would pay him
billions of dollars extra in case of a terrorist attack.
The weekend before the attack, the electric power in one or both
of the towers was shut off by the owner, supposedly to permit
upgrading of the computer network cables (which makes no sense).
With the security system thus disabled, unidentified people were
able to come in and set up the explosives (or at least the detonator
system for them) which later demolished the buildings.