There seems to be some confusion as to what did/didn't happen in this real life tragedy. So here's what IS known.
The black box made it clear that the passengers did not breach the cock pit, they never made it in. However, the pressure they put on the hijackers did more or less force the hijackers to crash the plane early.
In short, while the passengers didn't physically crash the plane, they are the reason it did crash. This is supported by statements from the US Government, people who spoke to the passengers on the phone while they were on the flight, and people doing flight control on the ground, and as I said, the Black Box. Like it or not, for those who are skeptical, this IS evidence, and this scenario has more backing than any of the other scenarios that have been suggested, which have nothing to back them up other than strange and questionable reasoning.