The National Transportation Safety Board published its first preliminary report Thursday on the Feb. 3 train derailment and chemical release in East Palestine, Ohio.

The report offers the most in-depth accounting of the events leading up to the derailment, detailing the number of freight cars involved, the equipment and materials those cars were carrying, and the hazardous materials officials ordered released in the aftermath as they raced to avoid a massive explosion.

EPA officials previously noted the freight train had been carrying at least five toxic chemicals when it derailed, including vinyl chloride, butyl acrylate, ethylhexyl acrylate, and isobutylene.

Thursday's report noted that NTSB investigators did return to the site of the derailment on Feb. 21 to examine each hazardous material tank car, document damage, and secure evidence for laboratory analysis.

NTSB investigators have also identified and examined the rail car that is believed to have initiated the the derailment. Surveillance footage captured by a nearby residence showed "what appeared to be a wheel bearing in the final stage of overheat failure moments before the derailment," the report says.

Both the wheelset and wheel bearing from the suspected railcar have been collected as evidence, and will be examined by engineers from the NTSB Materials Laboratory in D.C.

The NTSB investigation "is ongoing," the report noted.

The reported noted that future investigative activity will focus on the wheelset and bearing of the tank cars used by Norfolk Southern, as well as tank car design and damage, railcar maintenance procedures, use of wayside defect detectors, and railcar inspection practices.

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NTSB will also review the accident response to the Norfolk Southern derailment, including the decision to vent and burn of vinyl chloride, which has been linked to liver disease and some forms of cancer in instances of acute exposure.