The immediate reason behind a train derailment that caused toxic chemicals to spew into the surrounding area was an overheating wheel bearing that got to 253C above ambient temperatures, an official report has found.
The town of East Palestine, Ohio, US, has been left devastated by the February 3 derailment which led to evacuations as well as lasting fears of air and water contamination.
The preliminary report from the National Transportation Safety Board states that alarm messages were sent to the train's crew after the 23rd car, the one that was first to derail, "recorded the suspect bearing’s temperature at 253F (123C)" above ambient.
Anything from 170 to 200F requires the train to stop, the report stated.
It read: "After the train stopped, the crew observed fire and smoke and notified the Cleveland East dispatcher of a possible derailment."
The report also explained why authorities carried out a controlled burn of the toxic chemicals spewing out of the damaged cars.
Vinyl chloride was released into the air from five of those cars before crews ignited it to get rid of the highly flammable, toxic chemicals, creating a dark plume of smoke seen for miles around.
Emergency crews found that temperatures inside one of the five derailed tanks of vinyl chloride continued to rise due to a chemical reaction.
There were fears that unless stabilised the tank could have exploded.
The report states: "Responders scheduled a controlled venting of the five vinyl chloride tank cars to release and burn the vinyl chloride, expanded the evacuation zone to a 1-mile by 2-mile area, and dug ditches to contain released vinyl chloride liquid while it vaporized and burned."
There are still many questions left unaswered.
Further investigation by the NTSB will look at how the tank car design, wheelset and bearing affected the disaster.
They will also look at how responders dealt with the incident and whether maintenance procedures could have contributed.
Although residents were allowed to return to their homes, initially they were being advised to not drink the water. Officials have now said the water is safe to drink.
Thousands of dead fish were found in local streams in the wake of the derailment and reports of livestock dying sparked fears the contamination was far worse than initial reports suggested.
