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.

“Thus far, no concerning readings have been detected,” Pennsylvania Gov. Josh Shapiro said at a brief evening news conference about three hours after the venting and burning procedure began.

However, he urged Pennsylvania residents within a 2-mile (3.2 kilometer) radius of the derailment site to shelter in place and keep their doors and windows closed through the evening as a precaution in case of wind shifts.

Shapiro also said he had spoken to President Joe Biden, who had offered “the full support of the federal government” to Pennsylvania and Ohio.

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.