An official says investigators have yet to determine what caused a fiery derailment in northwest Iowa.

The Union Pacific train derailed early March 10 on a trestle bridge spanning a creek near Graettinger, about 160 miles northwest of Des Moines. The National Transportation Safety Board said in a preliminary report Thursday that 14 of the 20 derailed tankers released 322,000 gallons of ethanol. The fire burned itself out more than 2½ days later. No injuries were reported.

The report said the train was moving 30 mph at the time, the maximum for that stretch.

Rail samples and video from the lead locomotive's recorder will be analyzed. Board spokesman Peter Knudson said Monday that the derailment cause likely won't be determined for least a year.