I Call Bullshit
Frames from here, which is a frame by frame break-down of one of the second hit videos shown on CNN.
I almost got into a car accident today. A guy in a truck ahead of me braked suddenly, I was a little distracted by something, and I had to brake fast and swerve to avoid a collision.
I drive a medium-sized sedan, and the guy in front of me was driving a large pick-up truck.
I could not help but imagine what the collision would have been like if I hadn't stopped and swerved: my car would have slammed into the back of the truck and crumpled up, probably causing minimal damage to the back of the truck, and the truck would have been pushed forward a bit.
Even if I were going 500 mph, my car STILL would have crumpled into the back of the truck-- though I would have caused more damage to the truck and pushed it forward more. However, there is NO WAY that my car would bust THROUGH the truck without slowing or crumpling.
Now think about a situation where my car is the airplane shown above and the truck is the south tower, and remember airplanes are much more lightly constructed than cars.