The catastrophic Feb. 3 train derailment in East Palestine, Ohio, was caused by an overheated wheel bearing on the 23rd of 149 rail cars, according to a preliminary report issued Thursday by the National Transportation Safety Board.

The Norfolk Southern Railway train derailment led to a decision to burn more than a hundred thousand of gallons of highly toxic chemicals, leading to concerns the air and water in and around the town of 4,700 could be badly contaminated. While it was already believed the wreck was caused by a malfunctioning wheel, the NTSB report revealed new details.

“Surveillance video from a local residence showed what appeared to be a wheel bearing in the final stage of overheat failure moments before the derailment,” the report stated.

Video footage obtained by the Pittsburgh Post-Gazette showed that the train had traveled more than 20 miles with the malfunctioning rail car axle, which appeared to have been on fire or shooting sparks.

The tracks are equipped with HBD sensors to assess the temperature conditions of wheel bearings on passing trains, alerting crews if wheel bearings exceed 200 degrees Fahrenheit. The train that derailed passed three sensors before it derailed, with the doomed wheel bearing first registering 38 degrees above ambient temperatures, then 103 degrees, and finally 253 degrees above ambient temperatures, the report stated.

The HBD system transmitted a critical audible alarm message directing the crew to slow and stop the train. The engineer did so, but ​once the train stopped the crew saw fire and smoke and notified the Cleveland East dispatcher of a possible derailment.

By Feb. 5, the fire sparked by the overheated wheels bearing was under control, according to the report. But with five cars carrying 115,580 gallons of highly volatile vinyl chloride on the derailed train, and the temperature rising in at least one of them, the decision was made to execute a “controlled burn” of the chemicals, sending a black, toxic plume into the skies above East Palestine.

“This increase in temperature suggested that the vinyl chloride was undergoing a polymerization reaction, which could pose an explosion hazard,” the report stated. “Responders scheduled a controlled venting of the five vinyl chloride tank cars to release and burn the vinyl chloride.”