As residents are urged to flee, teams at the site of a burning derailed train in East Palestine, Ohio, are working to prevent a “catastrophic tanker failure” and explosion that could shoot deadly shrapnel up to a mile away, officials said.

The train was carrying hazardous materials when it derailed Friday, igniting a massive inferno and prompting evacuations and shelter-in-place orders amid concerns over air and water quality.

As the fire continued to burn for a third night Sunday, the threat escalated, and officials pleaded with those who remained in their homes in the one-mile radius around the crash site to evacuate quickly.

The explosion risk comes from a drastic temperature change inside one of the rail cars, warned Ohio Gov. Mike DeWine, who on Sunday activated the Ohio National Guard to deploy to the scene.

“We are at a risk now of a catastrophic failure of that container,” Fire Chief Keith Drabick said Sunday. “Measures are being taken to try and control that and prevent that from happening.”

In the meantime, people in the area must leave, the fire chief said. “We need you to leave now,” Drabick told residents.

While most people in the one-mile radius had already evacuated, more than 500 people had declined to leave their homes, according to local officials.

“There is a high probability of a toxic gas release and or explosion,” Columbiana County Sheriff Brian McLaughlin warned, “Please, for your own safety, remove your families from danger,” McLaughlin added.

Crew got a warning about a mechanical failure shortly before derailment

While the cause of the derailment remains under investigation, investigators learned there was a mechanical failure warning before the crash, NTSB Member Michael Graham said Sunday afternoon at a news conference.

“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 were also able to identify the point of derailment and found video showing “preliminary indications of mechanical issues” on one of the railcar axles, Graham said.

NTSB is still investigating when the potential defect happened and the response from the crew, which included an engineer, conductor and conductor trainee, Graham added.

Investigators have also requested records from Norfolk Southern, including track inspection records, locomotive and railcar inspections and maintenance records, train crew records and qualifications, Graham said.

Since the fire is still burning, investigators haven’t been able to walk around the crash site.