A broken rail was the probable cause of a fiery freight train derailment and subsequent bridge collapse in Tempe nearly two years ago, the National Transportation Safety Board said in its investigative report released Thursday.

Twelve railroad cars derailed after the train hit a bridge over Tempe Town Lake, causing it to partially collapse and spark a massive fire on July 29, 2020.

There were no fatalities, but several cars carrying hazardous materials overturned, leaking chemicals into the area. It cost Union Pacific $11 million to replace the bridge and make other repairs.

In its completed investigation, the NTSB said it determined that the probable cause of the derailment and bridge collapse was a broken rail located on the ballast deck portion of the wooden trestle approach about 30 feet from the steel bridge.

“Contributing to the severity of the derailment was the absence of an inner guard rail preceding the steel bridge structure, which allowed the derailed equipment to move laterally into the bridge structure and cause its collapse,” the report added.

The derailment occurred around 6 a.m. and involved 12 of the 97 railcars at the south end of the bridge, which was built in 1912.

Five of the derailed railcars were tankers carrying various hazardous materials and one released about 2,200 gallons of cyclohexanone, a flammable liquid that created a pool of hazardous material below the damaged bridge.

Derailed lumber cars, which remained on the bridge, caught fire during the derailment. One firefighter was treated for smoke inhalation at the scene.