All types of "theories" have been tossed around as to why two very large skyscrapers collapsed after being hit by commercial airliners. Even though the designers said that the buildings had been designed to withstand impacts by commercial aircraft this was for aircraft that were assumed to be flying slow and "lost" not aircraft at higher speeds and being used as "missiles". Anyone with common sense knows that controlled demolition is a stupid theory due to the amount of man power and time that that would've taken. The one theory that seems to stand up to scrutiny is design flaw. The designers and engineers that have talked about the WTC always mention how the towers were the only buildings that were built with that design. Name one building in the world that was designed the same way as the WTC was. The fringe element likes to post pictures of other steel buildings that burned but didn't collapse. Not one of those was hit by commercial aircraft before the fires started. Let's face it. Just because the original designers said that the buildings were built to withstand impact by aircraft doesn't make it true. It's happened before. Google the Tacoma Narrows Bridge and watch the video of what happened to the original bridge. That design flaw only killed one dog. The WTC was a lot worse.