The preliminary report from the NTSB indicates overheating of the train’s wheel bearing might have caused the toxic derailment (In the image: Smoke from Ohio train derailment)|Thunderlips36|CC BY-SA 4.0

Overheating of the Norfolk Southern train’s wheel bearing might have caused the derailment in Ohio that killed nearly 44,000 water species in surrounding rivers.

The preliminary report from the National Transportation Safety Board (NTSB) on Thursday indicates that the train crew received a warning to slow down on February 3, which it did. But during deceleration, the wheel bearings overheated and derailed 35 cars of the train—11 of which contained toxic chemicals.

The NTSB will also investigate the accident response that happened after the Ohio train disaster.