CNN
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After crews in East Palestine, Ohio, performed a controlled release of an unstable, toxic chemical that threatened an explosion at the site of derailed train, evacuated residents are still being urged to stay away Tuesday amid the wait for the fire to die down.

A boom was heard and a large plume of black smoke shot up as the release got under way Monday to drain vinyl chloride – the hazardous material the train was carrying in some of its cars – into a pit to burn it away.

“The detonation went perfect,” Scott Deutsch with Norfolk Southern said Monday. “We’re already to the point where the cars became safe. They were not safe prior to this.”

Still, there’s no timeframe on when it’ll be safe for residents of the area around the fiery derailment to return home, East Palestine Mayor Trent Conaway said Monday evening.

“We have to wait til the fires die down,” the mayor added.

The wreck continues to burn days after the train derailed Friday, igniting a massive inferno that built up pressure inside the hot steel and by Sunday evening was threatening a catastrophic explosion capable of spewing toxic fumes and firing shrapnel up to a mile away, authorities said.

The explosion fears triggered urgent calls for evacuations, with law enforcement going door to door pleading with residents that hadn’t already evacuated to flee.

The release was planned Monday to alleviate the risk of an uncontrolled explosion and officials expanded the evacuation zone surrounding the fiery derailment to a 1-mile-by-2-mile area that included part of Pennsylvania.

Ohio Gov. Mike DeWine had warned that although it was a controlled release, the operation could spew fumes into the air that can be deadly if inhaled and leave people with skin burns and serious lung damage.

After the breach, officials detected “slightly elevated” readings of the phosgene and hydrogen chloride in the burn area and “only one minor hit for the hydrogen chloride downwind of the burn area” within the exclusion zone, James Justice of the US Environmental Protection Agency said Monday evening.

Justice said those readings were expected in the aftermath of the controlled release.

As for the village’s water supply, no impacts to the waterway were detected as of Monday evening, an Ohio Environmental Protection Agency official said.

A team will continue to monitor the air and water quality in the area, officials said.

Pennsylvania Gov. Josh Shapiro, who had also called for evacuations, said Monday evening that air and water quality is being monitored closely and no concerning readings had been detected so far.

He however told Pennsylvanians who live within 2 miles of the East Palestine derailment to continue to shelter in place with their windows and doors closed Monday evening.

The plan now is to wait for the fires at the train wreck to go out on their own, wait for it to cool down and – if no dangers in the air are detected – go in to begin moving the cars off and tracks and into a safe location to be looked at by investigators, Deutsch said.

Meanwhile, the derailment has upended life in East Palestine, a village of about 5,000 people near the Pennsylvania-Ohio border. Schools have been closed and some residents haven’t been home since the initial evacuation orders Friday.

When the Norfolk Southern train crashed in East Palestine, about 10 of 20 cars carrying hazardous materials derailed.

One rail car carrying vinyl chloride became a particular concern when its malfunctioning safety valves prevented the release of the chemical inside, a Columbiana County Emergency Management Agency official and a Norfolk Southern spokesperson told CNN.

Deutsch said that the valve not working meant that “the car’s just building pressure inside the steel shell and that’s a problem.”

“It won’t get worse again because the cars were all breached,” Deutsch said after the controlled release. “There’s no pressure now in the cars.”

Vinyl chloride is a man-made chemical used to make PVC and it burns easily at room temperature. It can cause dizziness, sleepiness and headaches; and has been linked to an increased risk of cancer in the liver, brain, lungs and blood.

Breathing high levels of vinyl chloride can make someone pass out or die if they don’t get fresh air, the Ohio Department of Health said.

On Monday afternoon, charges were used to blow small holes in each rail car, allowing the vinyl chloride to spill into a flare-lined trench.

The trench was still on fire as of 7 p.m. Monday, Deutsch said at the news conference.

While the cause of the derailment remains under investigation, National Transportation Safety Board Member Michael Graham said Sunday that there was a mechanical failure warning before the crash.

“The crew did receive an alarm from a wayside defect detector shortly before the derailment, indicating a mechanical issue,” Graham said. “Then an emergency brake application initiated.”

Investigators also identified the point of derailment and found video showing “preliminary indications of mechanical issues” on one of the railcar axles, he added.

The NTSB has requested records from Norfolk Southern and is investigating when the potential defect happened and the response from the train’s crew, which included an engineer, conductor and conductor trainee, Graham added.