Well, there was an autopsy. It did not find a conclusive reason for his death. They found no drugs, no alcohol in Mr. Dziekanski's system, some sign of long term alcohol abuse. There is also no mention of finding that he died from unnatural restraint (the knee to the neck) or the multiple taser shots.
All we've got are the stories, and they tell a sad tale of pretty much everyone in the case doing something, but not enough. Mother saved up money to bring son over. Son was very apprehensive about flying and cancelled at least one flight. Mother told son misinformation about where she'd meet him. Mother made it to airport, but was misinformed that no one was left there, and she departed.
Just incredibly sad, and incredibly unneccesary. The cops were just cops, they could have been better, but they interpreted what they saw in front of them without putting a lot of thought or concern into it. They didn't see a badly confused, lonely man with no communication skills, they saw someone taking his frustrations out physically. They didn't imagine a lonely son, they saw a man holding a stapler out as some kind of weapon.
Everyone in the story acted without much foresight, without backup plans, without any imagination.
It didn't have to happen, and all but one of the actors have to live with the consequences.