Eye witness acconts are just a part of the fact gathering process as well as soot and debris on the runway, etc. etc.
It is the linking of these facts that becomes the basis of a causal chain. If the hypothesis being tested to propose a complete link of the causes of an accident is challenged by any fact, then that fact is examined in detail for its validity (is it a could be? or scientifically accepted). Is it verrifieable by more than one means?. Just because 2 or 3 people say there was a fire earlier based on observation doesn't mean they were seeing the same evidence. when examined in detail they are often not sure what they saw and precisely when, but vaguely remember their conclusions.
On the other hand the exact location of a mark on the runway is verifiable by photos and GPS as are metalurgical findings. So often "some" eye witness reports are not verifiable by other means and are ulimately discarded from the analysis section of the accident investigation.