An oil train accident near Casselton, N.D., last month started when an oncoming grain train derailed, according to a preliminary report by the National Transportation Safety Board.
The NTSB report, released Monday, confirms that 13 cars of the westbound grain train derailed on Dec. 30. The eastbound crude oil train on an adjacent track then crashed into a derailed car.
Both trains were traveling slower than the posted speed limit for the track, officials said.
The collision derailed 21 cars of the oil train, causing an estimated 400,000 gallons of oil to spill. Most of it burned when the tanker cars caught fire and exploded, sending fireballs high into the sky.
There were no injuries, but emergency responders urged about 1,600 local residents to evacuate that day. Damage is estimated at $6.1 million.
The NTSB is still analyzing a broken axle, two train wheels and incident recorders and onboard video from the locomotives involved in the crash.
A final accident report will be released in about a year.