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On a cold, overcast afternoon in January 2003, two tanker trucks backed up to an injection well site in a pasture outside Rosharon, Texas. There, under a steel shed, they began to unload thousands of gallons of wastewater for burial deep beneath the earth.

The waste – the byproduct of oil and gas drilling – was described in regulatory documents as a benign mixture of salt and water. But as the liquid rushed from the trucks, it released a billowing vapor of far more volatile materials, including benzene and other flammable hydrocarbons.

The truck engines, left to idle by their drivers, sucked the fumes from the air, revving into a high-pitched whine. Before anyone could react, one of the trucks backfired, releasing a spark that ignited the invisible cloud.

Fifteen-foot-high flames enveloped the steel shed and tankers. Two workers died, and four were rushed to the hospital with burns over much of their bodies. A third worker died six weeks later.

What happened that day at Rosharon was the result of a significant breakdown in the nation’s efforts to regulate the handling of toxic waste, a ProPublica investigation shows.

The site at Rosharon is what is known as a “Class 2” well. Such wells are subject to looser rules and less scrutiny than others designed for hazardous materials. Had the chemicals the workers were disposing of that day come from a factory or a refinery, it would have been illegal to pour them into that well. But regulatory concessions won by the energy industry over the last three decades made it legal to dump similar substances into the Rosharon site – as long as they came from drilling.