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Investigators said there was no imminent sign of the chemical reaction, known as polymerization, that would’ve cause tank cars to explode. They said Norfolk Southern and its contractors “misinterpreted and disregarded evidence” in advocating for what’s known as a vent and burn operation, which involved blasting holes in tanker cars, draining the vinyl chloride into pits in the ground and setting it on fire.

“Norfolk Southern and its contractors continued to assert the necessity of a vent and burn, even though available evidence should have led them to re-evaluate their initial conclusions,” Paul Stancil, a hazardous materials investigator at the NTSB, said Tuesday.