(NEW YORK) — Residents near the Ohio-Pennsylvania state line who were told to evacuate after tanker cars derailed in a fiery crash days ago have still not been allowed to return home.

Crews conducted a controlled release and burn of toxic chemicals from five of the derailed cars that were in danger of exploding on Monday afternoon. A large ball of fire and plume of black smoke could be seen billowing high into the sky from the smoldering derailment site in East Palestine, Ohio, as the controlled burn took place.

As of Monday night, a 1-mile mandatory evacuation zone remained in place around the site, with no timeframe for when residents will be let back in, according to East Palestine Mayor Trent Conaway.

East Palestine is a small village in northeastern Ohio, near the border with Pennsylvania. It is home to roughly 4,700 residents, about half of whom had been warned to leave over the weekend before officials decided on Monday to conduct the controlled release.

About 50 cars of a Norfolk Southern Railroad train, traveling from Illinois to Pennsylvania, derailed in East Palestine on Friday at around 9 p.m. ET. Ten of the derailed cars were transporting hazardous materials, five of which contained vinyl chloride. No injuries were reported, officials said.

According to the National Transportation Safety Board, two videos show preliminary indications of mechanical issues on one of the car’s axle. The train’s emergency brake was activated after crews said an alarm went off.

Efforts to contain a fire at the derailment site stalled on Saturday night, as firefighters withdrew from the blaze due to concerns about air quality and explosions.

Forced evacuations began in East Palestine on Sunday night and by Monday, residents in a 1-mile by 2-mile area surrounding the site — which includes parts of Ohio and Pennsylvania — were ordered to evacuate immediately.

“The vinyl chloride contents of five rail cars are currently unstable and could potentially explode, causing deadly disbursement of shrapnel and toxic fumes,” the office of Ohio Gov. Mike DeWine said in a statement Monday.

The controlled release and burn went “as planned,” according to Pennsylvania Gov. Josh Shapiro.

“Thus far, no concerning readings have been detected,” Pennsylvania Gov. Josh Shapiro said during a press conference on Monday evening, about three hours after procedure began. “For now, out of an abundance of caution, Pennsylvanians who live within two miles of East Palestine, where this derailment occurred, should just continue to shelter in place this evening and keep your windows and your doors closed.”

Crews monitoring air quality “have not seen anything” unexpected, according to James Justice of the U.S. Environmental Protection Agency.

“So far, so good,” Justice said at the press conference. “And we’re going to continue to monitor until the fire’s out.”

In a statement to ABC News, Norfolk Southern Railroad also called Monday’s controlled release a success and said materials were burning off according to plan.

“We have been, and will continue, monitoring air quality with the Ohio EPA,” the rail operator added. “Remediation work at the site can now safely continue.

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