About 400,000 gallons of crude oil spilled from 18 rail cars after after a Dec. 30 derailment near Casselton, N.D., the National Transportation Safety Board said in a preliminary investigation report Monday.
An ensuing explosion sent a massive mushroom cloud of fire above the prairie and forced the evacuation of 1,400 residents.
Damage was estimated at $6.1 million, the NTSB said.
The accident occurred when a BNSF Railway grain train derailed on the westbound tracks, obstructing the eastbound tracks less than a minute before the 106-car oil train arrived.
The NTSB's preliminary report shed little new light on what may have caused the grain train to derail. Twenty-one cars on the oil train derailed, including 20 carrying crude and one carrying sand ballast.
Both trains were under the 60 mph speed limit for freights. The oil train was traveling at 43 mph when the crew triggered emergency brakes, and had slowed to 42 mph when it crashed into a car obstructing its track. The grain train was traveling about 28 mph when it derailed.
The cause of the grain train's derailment remains under investigation. The NTSB has shipped a broken axle and two wheels from that train to its laboratory in Washington for analysis.
In an earlier statement, investigators said they could not be sure whether the broken axle caused the crash or resulted from it. Investigators also said in the earlier statement that the derailment occurred on or near a switch to a side track, although the switch appeared to be properly aligned.
In the days after the accident, the Pipeline and Hazard Material Safety Administration issued a safety alert that the Bakken crude being hauled by the BNSF train was more flammable than expected. The agency, which is part of the federal Transportation Department, said it was conducting further investigation into the crude's flammability.
Over the last six months, there have been five major rail accidents in North America, including one Wednesday in New Brunswick, Canada, in which crude hauled in tanker cars burned or exploded.