Agreed, WillowRun - not saying I agree with the approach, just that that's the way it's panned out here.
In an accident, there is always the alignment of the 'holes in the swiss cheese' to discover - which takes time, and painstaking sifting of all the evidence, etc.
In this case, it looks like all that needs to be known to draw reasonable conclusions is already there - plus, it seems like it wasn't an 'accident' - rather a deliberate act.
That one mere fact changes the whole philosophy of the investigation - there aren't any 'holes in the cheese' to line up. It appears to have been a singular cause, by a singular act. Thankfully, an exceptionally rare one (as per my previous post);.