BBC Documentary. I have seen other videos and read other articles that address this topic. I think this one is a good summary of the building design, materials, and forces that all contributed to the collapse of the towers.
I hesitated to post this because of the numerous conspiracy theories about a government controlled demolition, a cabal by the Jews etc. This is NOT a forum to discuss conspiracy theories! I will make one comment here about controlled demolition. If you watch the collapse of the towers, you can easily see that the collapse is from the top down, each floor collapsing the one below it. A controlled demolition is done from the bottom up, or multiple points, and the explosions before collapse are clearly visible and audible.
Believe me, I have heard every conspiracy theory over the years about the twin towers, and Tower Seven. They have been addressed.
Don’t comment unless you have first watched the entire documentary, and then directly address the points made in this video by the building structural engineers, fire experts etc.
Stella's Place 
I’ve watched this a couple of times. What stuck me were the clips of interviews with the architect who designed the towers. At one point, you can see the internal agony in his face, as he felt he was partly responsible for the towers collapsing. One detail in particular. They took into account an accident such as a plane hitting the towers – the largest plane in service at the time, even. They did not take into account the effect of the fuel igniting.
It was no surprise to hear that he died only a year later.
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