Crews released toxic chemicals into the air from five derailed tanker cars that were in danger of exploding Monday and began burning it after warning residents near the Ohio-Pennsylvania state line to leave immediately or face the possibility of death.

Flames and black smoke billowed high into the sky from the derailment site late in the afternoon, about an hour after authorities said the controlled release would begin.

The slow release of vinyl chloride from five rail cars into a trough that was then ignited created a large plume above the village of East Palestine but authorities said they were closely monitoring the air quality.

Ohio Gov. Mike DeWine earlier ordered evacuations in the area of the derailment that has been smoldering since Friday night. Authorities believed most, if not all, residents in the danger zone had left but they were knocking on doors one more time before releasing the vinyl chloride inside the cars, he said.

“You need to leave, you just need to leave. This is a matter of life and death,” DeWine said at news conference.

Officials warned the controlled burn would send phosgene and hydrogen chloride into the air. Phosgene is a highly toxic gas that can cause vomiting and breathing trouble and was used as a weapon in World War I.

Scott Deutsch of Norfolk Southern Railway earlier said doing this during the daytime would allow the fumes to disperse more quickly and prevent the rail cars from exploding and sending shrapnel and other debris from flying through the neighborhood.

“We can’t control where that goes,” he said.

The process involves using a small charge to blow a hole in the cars, allowing the material to go into a trench and burning it off before it’s released in the air, he said. The crews handling the controlled release have done this safely before, Deutsch said.

Forced evacuations began Sunday night in East Palestine after authorities became alarmed that the rail cars could explode after a “drastic temperature change” was observed in a rail car.

Residents were packing overnight bags, loading their pets into cars and searching for hotel rooms Monday morning. Police in the village moved out of their communication center as the threat of an explosion increased.

Police cars, snow plows and military vehicles from the Ohio National Guard blocked streets leading into the area.

Mayor Trent Conaway, who declared a state of emergency in the village, said one person was arrested for going around barricades right up to the crash. He warned people to stay away and said they’d risk arrest.

“I don’t know why anybody would want to be up there; you’re breathing toxic fumes if you’re that close,” he said.

Read the full article about derailed tankers in Ohio by Patrick Orsagos and John Seewer at Time.