EAST PALESTINE, Ohio (AP) - It's unclear when evacuated residents might be able to return home to the area where officials released and burned toxic chemicals from the wreckage of a derailed train, Ohio Gov. Mike DeWine said Tuesday.

Residents near the site in East Palestine, close to the Pennsylvania line, were ordered beforehand to leave because of the risk of death or serious injury from toxic fumes. Flames and black smoke billowed into the sky Monday evening when crews released and burned vinyl chloride from five derailed tanker cars that were in danger of exploding.

DeWine said on "Fox & Friends" on Tuesday morning that officials are closely monitoring the air quality outside the immediate area and that it's "so far, so good." Ohio National Guard members wearing protective gear are expected to be sent into the area closer to the site with sensors to check the air, he said.

Residents just outside the evacuation zone in East Palestine and in neighboring Beaver County, Pennsylvania, were urged to stay indoors as a precaution.

Authorities believed most, if not all, residents in the danger zone had left. They went through the area three times trying to get people out before releasing the vinyl chloride, DeWine said.

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.

Doing the release during the daytime allowed the fumes to disperse more quickly and prevented the rail cars from exploding and sending shrapnel and other debris flying through the neighborhood, said Scott Deutsch, of rail operator Norfolk Southern Railway.

"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 had done this safely before, Deutsch said.

About three hours into the procedure, Norfolk Southern issued a statement saying that experts and first responders had breached the rail cars, chemicals were burning off and the cars were expected to drain for several more hours.

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

Pennsylvania Gov. Josh Shapiro said that the evacuation zone included about 20 Pennsylvania residences, and that he was told residents within a mile (1.6 kilometers) of the controlled burn had left.

About 50 cars, including 10 carrying hazardous materials, derailed in a fiery crash Friday night while traveling from Madison, Illinois, to Conway, Pennsylvania, according to Norfolk Southern and the National Transportation Safety Board. No injuries to crew members, residents or first responders were reported.

Federal investigators say a mechanical issue with a rail car axle caused the derailment.

Five derailed cars were transporting vinyl chloride, which is used to make the polyvinyl chloride hard plastic resin in plastic products and is associated with increased risk of liver and other cancers, according to the federal government's National Cancer Institute.

Forced evacuations began Sunday night after authorities became alarmed the rail cars could explode after a "drastic temperature change" was observed in one car.

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