Bruce Crummy / AP
A fireball goes up at the site of an oil train derailment Monday, in Casselton, N.D.
The fiery explosion of an oil train in North Dakota was caused when it slammed into a single car of a grain train that had derailed on to its tracks, investigators said Tuesday.
An evacuation order was lifted Tuesday afternoon for the 2,500 residents of nearby Casselton, meaning the site of Monday afternoon's explosion was finally safe for federal investigators to enter. No one was injured, but the impact ignited an enormous fireball and choked the area with black, sooty smoke.
A spokeswoman for the railroad that operated both trains, BNSF of Fort Worth, Texas, had said initial indications were that the grain train knocked several cars of the oil train off its track.
But Robert Sumwalt, a member of the National Transportation Safety Board who's overseeing the inquiry, told reporters late Tuesday afternoon that just one car of the grain train came to rest on the other track, and the oil train "struck that one car."
Investigators are optimistic that they can retrieve a great deal of information from event and video recorders on both trains, Sumwalt said.
Two of the event recorders on the oil train were destroyed, but the third one — on the rear locomotive — survived intact. Some of the information from the surviving event recorder could be available as early as Wednesday, Sumwalt said.
Video recorders on both trains were streaming their data in real time, so it's already retrievable, he said.
"We think we will have multiple sources of data," he said.
Sumwalt said the 112-car grain train derailed for unknown reasons as it was carrying soybeans from Royal, Neb., to Rivergate, Wash.
It's unknown whether the conductors of the oil train — which Sumwalt said was rated at the highest hazard level for flammability — even knew there was giant grain car on the tracks in front of them. The crews of the two trains won't be interviewed until Thursday, he said.
The oil train, which was traveling from Fryburg, N.D., to Haiti, Mo., comprised three locomotives and 106 un-upgraded DOT-111 tank cars — the same kind of cars that blew up when a Montreal, Maine & Atlantic oil train derailed in Quebec in July, killing 47 people.
In September, NBC News Investigations reported that the DOT-111 has a serious design flaw and can split in an accident, potentially turning a derailment into a fiery catastrophe.
Other questions the NTSB is hoping to answer:
- Was the crew of the grain train able to send an emergency message that could have alerted the other crew?
- Are there any visible wheel marks in the earth to mark the point of derailment?
- Could the grain train have developed any mechanical faults that caused the sudden derailment? That information could come from an automated data recorder 4 miles back up the track that should have registered "hot spots" as the train passed, Sumwalt said.
"It's like a jigsaw puzzle," he said. "We're going to take all of the information and piece it together."
This story was originally published on Tue Dec 31, 2013 1:25 PM EST