I am going to say that the planes were the main reason

The planes crashing into them. Aviation fuel burns very hot.

The four implosions on each floor of the twin towers, I am not including Tower 7. I don't believe any planes crashed into the towers as I have investigated this for over four years.

I hope one day to know, but engineering calculations cannot support the conclusion that the jet impact plus fuel fire were sufficient to cause more damage that we saw prior to the collapse. As I watched the fire I was completely shocked to see one building collapse in a vertical demolition manner. Never happened before or since with two exceptions; the second tower and building 7. There are people who were on the impacted floors who escaped after many minutes exposure to the fire, some with severe burns. Their survival confirms that the temperature was not sufficient to weaken the steel several minutes after impact when the amount of fuel available was greatest and therefore should have been hottest. Here is a website which contains calculations of the temperature increase to be less than 495 degrees Fahrenheit and states that steel is not weakened until 1100 degrees Fahrenheit. http://911research.wtc7.net/mirrors/guardian2/wtc/how-hot.htm If the twin towers were flawed in their design/construction causing collapse at this low temperature, then what caused the collapse of building 7 which was struck by a small amount of debris and had a small fire?

Structural failure combined with the Force of Gravity.

Foul play. The falling of the Towers very much looks like controlled demolition. Unfortunately, we'll never know the truth

When the planes crashed into the towers, it caused some structural weakness, but not enough to collapse the towers. As the heat built up inside, the metal began to weaken. It did not have to completely weaken because there were many floors above putting an awful lot of stress on the metal structure, which was already weakened due to the crash. At some point, the metal began to give and the reaction went very fast after that. All the floors above went straight down, each adding it's weight to the one below compacting to create a giant pile driver smashing the building to little pieces. I have no clue about building 7. It looked like a professional demo job, but those usually involve removing a lot of interior structural supports, so I don't really have an answer.

The initial crash weakened the local structural capabilities The subsequent fires further weakened structural integrity and caused the tower to collapse Following the first collapse, the tower weakened the structures of the surrounding towers

I hear that, the impact from the planes knocked down the fireproof paint from the structural metal beams and the fuel melted were they joined. Then the floors collapsed on each others.

shaped explosive Demolitions charges and carefully pre-weakened supports. isn't is convenient that G.W. wanted to invade Iraq and he got the perfect excuse during his term in office?

Who cares about the buildings, they can always be rebuilt. What we should care about is the 3000+ lives lost and their families. Those cannot be replaced.