During a Sunday appearance on ABC News' "This Week" with George Stephanopoulos, National Transportation Safety Board lead investigator Robert Sumwalt shared the most current information on whether Amtrak Train 188 may have been struck by an object or projectile.
RELATED STORY: Was Amtrak Train 188 struck by an object before derailing?
Sumwalt said that the NTSB interviewed dispatchers and listened to dispatch tape, but "heard no communications at all from the Amtrak engineer to the dispatch center to say that something had struck his train."
The comment by Sumwalt comes as investigators are looking to explain what caused a circular pattern of damage found on the locomotive's windshield after the accident.
The NTSB has asked the FBI to help it examine the damage to the windshield, which is about the size of a grapefruit.
"This idea of something striking the train, that's one of the many things we are looking at right now," he said.
Sumwalt told CBS that investigators have all but ruled out the idea that a gunshot caused the damage to the windshield.
On Friday, investigators interviewed two assistant conductors and the engineer who was driving the train when it derailed.
One conductor had told investigators she may have heard the engineer say over the train's radio that something hit the windshield, Sumwalt said. But so far officials have found no independent corroboration of such a communication.
Sumwalt emphasized that the investigation is still in the fact-finding phase of a thorough process, adding that positive train control and inward-facing video cameras are critically important safety improvements that should be given priority by the U.S. passenger rail service.