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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.

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, where the oil train "struck that one car."

Sumwalt said the 112-car grain train derailed for unknown reasons as it was carrying soybeans from Royal, Neb., to Rivergate, Wash.

It's also unknown whether the conductors of the 106-car 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.

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."