Keep in mind I "SUSPECT" that this is what happened. I don't absolutely believe it, just suspect it.

Radical Muslim loyalists were distraught about US Imperial policies in the East, so they planned a hijacking. It was to be a regular hijacking, with the usual demands in exchange for the lives of the passengers. I suspect that "some" of the 19 hijackers listed were involved.

The US/Israeli intelligence complex caught wind of the plot...Then Along Came A Spider

Some people saw this as a chance to beef up their budget and they made a conscious choice to not only pave the way for it to happen, but, to change the outcome of the hijacking.

There were remote control devices installed in the planes and transponders and thermite based demolition charges placed at the WTC 1, 2, and 7. The reason why they destroyed building 7 was because that was the base information center for this to happen, thus, it and the evidence needed to be destroyed.

I suspect the hijackers had no idea they were on a suicide mission. The first two planes were guided to their destinations by remote control.