(NewsNation) — Emergency crews began releasing toxic chemicals into the air from derailed tanker cars that were in danger of exploding Monday after warning hundreds of Ohio residents to leave immediately or face the possibility of death.

Fearing a “catastrophic” explosion, officials conducted the controlled chemical release to reduce the risk of hazardous fumes and shrapnel being released into the air. The process involved blowing a small hole in the freight cars with a controlled charge, allowing the material to go into a trench rather than burning in the air.

Ohio Gov. Mike DeWine earlier ordered evacuations in the area of the derailment that has been smoldering since Friday night. Authorities believe 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 press 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 chemical weapon in World War I.

“Some of the material is now burning off consistent with expectations from the earlier models, and is expected to drain for a short number of hours,” Norfolk Southern Railway said in a written statement Monday. “We have been, and will continue, monitoring air quality with the Ohio EPA. Remediation work at the site can now safely continue.”

The crash site is very close to the state line, and the evacuation area extends into a sparsely populated area of Pennsylvania. The Associated Press reported about half of the 4,800 residents in East Palestine had been warned to leave over the weekend before officials decided on Monday to use the controlled release.

Forced evacuations began Sunday night in the village of 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.

About 50 cars, including 10 carrying hazardous materials, derailed in a fiery crash Friday night, according to rail operator Norfolk Southern and the National Transportation Safety Board. No injuries to crew, residents or first responders were reported.

It is not clear what caused the train to veer off its tracks, but a rail car axle may be to blame, according to federal investigators from the National Transportation Safety Board.

The Associated Press contributed to this report.