Crews successfully completed a controlled breach of five derailed train cars Monday to reduce an explosion threat near the Ohio-Pennsylvania border, authorities said.

The “controlled release” of vinyl chloride began Monday afternoon amid the threat of a possible major explosion from the wreckage of a train carrying hazardous materials that went off the tracks Friday night in East Palestine, Ohio.

A loud boom followed by flames and a plume of black smoke spreading into the sky were seen at the derailment site Monday afternoon. According to a news release from Norfolk Southern Railway, the breach of several rail cars was completed successfully.

Norfolk Southern said some of the hazardous material is now burning off and is expected to drain for a short number of hours. 

Authorities said Monday night that the situation will be monitored overnight but so far environmental agencies have detected “nothing alarming” in the air and water quality.

“Thus far, no concerning readings have been detected,” said Pennsylvania Gov. Josh Shapiro at a brief evening news conference hours after the controlled release began. 

Out of an abundance of caution, officials have asked people within two miles of the evacuation zone to remain indoors and continue to shelter in place. Shapiro also urged residents within the two-mile radius to keep their doors and windows closed through the evening as a precaution in case of wind shifts.

Scott Deutsch, Norfolk Southern’s regional manager for hazardous materials, said the daytime release would lower the risk of rail cars exploding and allow the fumes to disperse faster. 

The Ohio National Guard and law enforcement officials blocked off roads Monday into East Palestine as hundreds of residents were warned of the dangers.

Authorities in the community of 4,761 people were enforcing what had previously been a strongly recommended evacuation zone within a 1-mile radius of the site where 50 cars came off the tracks Friday night, according to the village of East Palestine. 

Within a two-hour period Sunday night, Ohio Gov. Mike DeWine said “a drastic temperature change” posed the threat of a “catastrophic tanker failure” that could send potentially deadly shrapnel up to a mile away.

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