A fuel-air device is what I was led to believe that a thermobaric bomb was.

They do exist but have limited use on 'soft' targets. It produces a very large fireball and an overpressure that can knock down barracks buildings, kill people, clear away tress (once you wait for the forest fire to move on), scatter and burn parked aircraft.

I saw a video of a test of one years ago.

Small detonation occurs about 100 feet agl dispersing a quantity of three flammable liquids, one of which , IIRC, was diesel fuel. The fuels expand in the open air for a short time allowing the fuel to form a 'cloud' and then an ignitor sparks this to life.

The result is a huge fuel explosion at an optimal height to cause blast and fire damage over a large area.

The 9/11 CT's want this to be the explosion in the basement levels.
They fail to notice that a large quantity of kerosene falling down the elevator shafts, propelled by the detonation of partially vaporized kerosene on the upper floors would essentially be the same as a fuel -air bomb of human manufacture.

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"Man has always found it easier to sacrifice his life than learn the multiplication table." - Somerset Maugham