Sure they do. But they get evidence to back up the witnesses, or the witnesses' testimony can be shot down by a good defense attorney.
No forensic evidence revealed the presence of explosives or a bomb on this flight.
But, the conspiracists don't care about facts.
Were do you get this stuff sink?
A credible witness is invaluable in any investigation. Two credible wits as in this case would be or should be critical to any investigative summary. If three wits all stated, "the tractor trailer ran through the red light and struck the honda car", would you dismiss this, or consider is unreliable as you stated above?
"No forensic evidence revealed the presence of explosives or a bomb on this flight."
Well then, lemme ask you, how does the fire/explosion correlate to the tail and engines coming off the plane?