The helicopter crew was primed to mistake the camera for a weapon, and then jumped straight to engagement without doing to much checking. I'm not sure exactly what the helicopter crew saw, but I get the impression that video is a low res black and white recording of what they where looking at. And what I saw in the video would justify a closer look but not an engagement. That part is an ugly accident of war, if you support a war you have to accept that such accidents will happen.
The cover up after looks even uglier. The military investigators must have quickly realized there was no RPG and no planned attack. I would assume a Reuters photography team in Iraq had some bodyguards, so there probably where some small arms to find, but nothing to justify the attack. Plus, it must have been clear quickly that most, if not all, of the targets where unarmed civilians.
Looking at the video it was obvious they where not really preparing for an attack, they where all clustered around the cameraman when the helicopter crew decided they where preparing to open fire. And what would they be shooting at? The US forces where several streets away in the other direction and they didn't seem to notice the helicopter either. Given the time delay between when the crew seemed to pull the trigger and the shells hit the ground, the helicopter seems to have been some distance away.
Jay