Let's look closer
It has not been established that the two "drifters" in custody are the actual murderous snipers. There are a number of - shall I say- irregularities emerging in the pertinent details of this case, so I think we need to wait and see.
Nor has is been established that those who were reporting a white van, panel truck, etc. were mistaken in their identification of the vehicles mentioned.
Rather, their mistake was in making a connection between the vehicle(s) they saw and the perpetrators of the killings.
By contrast, in the case of F. 587, I don't think there's any question, at least in my mind, that the plane the eyewitnesses have described is the one which actually crashed.
Apples and oranges.
sp