I recommend researching for yourself, not one answer on here will be long enough to completely explain the complexity of those that conspire to dominate the worlds politics, even after ~7 years of research, there are many questions left unanswered.
Google "illumanti structure" and go from there.
This is the most realistic structure I've seens so far: http://bit.ly/g37p3H
Charity stops people from solving their own problems, give a man a sandwich bar and he will eat for a day, tell him to get off his * and do something...
Only exception is people are physically unable to solve their own problems such as disabled.
Even if fire damage could bring a build down it would take ages and the structure would not just suddenly turn into powder, would be a very slow/gradual, all-over-the-place fall.
Also explain the molten steel as well...since jet fuel isn't capable of that, only "weaking the steel".
How it was (most probably) done:
1. Don't build the buildings properly in the first place (X-beams left out due to rushing).
2. Get some R/C planes.
3. While before, fill the buildings with thermite & igniters so when ignited in time they cause the structure to fall in on itself to minimize outside damage.
4. Drop the air-defenses for the time needed to do the job (eg: turn off SAMs and divert interceptors somewhere else).
5. Have a training exercise running at the same time, to confuse the lower echelons of the Air force.
5. Do the job.