EAST PALESTINE, Ohio (AP) - Officials were working to prevent any major explosion from the smoldering wreckage of an Ohio train derailment near the Pennsylvania state line as the governor activated the Ohio National Guard to help the village where many residents had to evacuate.

The village of East Palestine said Monday morning that authorities would begin to enforce what had previously been a strongly recommended evacuation zone within a 1-mile (1.6-kilometer) radius of the crash site.

Officials had warned hundreds of nearby residents who had declined to evacuate to do so Sunday night, saying a rail car was at risk of a potential explosion that could launch deadly shrapnel as far as a mile.

They warned of "the potential of a catastrophic tanker failure" after a "drastic temperature change" was observed in that rail car, according to a statement from Ohio Gov. Mike DeWine's office. It did not specify what was in that car or whether it was among those that had been carrying hazardous materials.

The statement said the Guard will be assisting local and state authorities already on scene in the village, which has closed roads, schools and businesses and even evacuated its police communications center, promising in a Facebook post that 911 services wouldn't be affected.

About 50 cars derailed in a fiery crash Friday night as the train carried a variety of products from Madison, Illinois, to Conway, Pennsylvania, rail operator Norfolk Southern said. No injuries to crew, residents or first responders were reported.

Federal investigators say the cause was a mechanical issue with a rail car axle. The three-member train crew received an alert about the mechanical defect "shortly before the derailment," Michael Graham, a board member of the National Transportation Safety Board, said Sunday. Graham said investigators identified the exact "point of derailment" but the board was still working to determine which rail car experienced the axle issue.

A preliminary investigative report is expected in the next month or so.

East Palestine officials have said emergency responders were monitoring but keeping their distance from the fire, and that remediation efforts could not begin while the cars smoldered.

Mayor Trent Conaway, who declared a state of emergency in the village, said one person was arrested for going around barricades right up to the crash. He warned people to stay away and said they'd risk arrest.

"I don't know why anybody would want to be up there; you're breathing toxic fumes if you're that close," he said, stressing that monitors of air quality away from the fire showed no levels of concern and the town's water is safe because it is fed by groundwater unaffected by some material that went into streams. Environmental protection agency crews were working to remove contaminants from streams and monitor water quality.

Sheriffs went door-to-door Sunday to count residents remaining and urge people within the evacuation area to leave.

Norfolk Southern said 20 of the more than 100 cars on the train were classified as carrying hazardous materials - defined as cargo that could pose any kind of danger "including flammables, combustibles, or environmental risks."

The NTSB said only 10 cars carrying hazardous materials derailed, and five of them were carrying vinyl chloride, not 14 as was said earlier. Officials stressed late Saturday that they had not confirmed the release of vinyl chloride other than from pressure release devices operating as designed.

Vinyl chloride, used to make the polyvinyl chloride hard plastic resin in a variety of plastic products, is associated with increased risk of liver cancer and other cancers, according to the federal government's National Cancer Institute.

"Short-term exposure to low levels of substances associated with the derailment does not present a long-term health risk to residents," according to a "Frequently Asked Questions" post on the village Facebook page. "Vinyl chloride and benzene may cause cancer in people exposed in the workplace to high concentrations for many years; however, there is no indication that any potential exposure that occurred after the derailment increases the risk of cancer or any other long-term health effects in community members."

Officials said Sunday afternoon that cars involved also carried combustible liquids, butyl acrylate and residue of benzene from previous shipments, as well as nonhazardous materials such as wheat, plastic pellets, malt liquors and lube oil.

The evacuation order covered homes of 1,500 to 2,000 of the town's 4,800 to 4,900 residents, but officials said it was unknown exactly how many were actually affected. Most of those who had gone to an emergency shelter were no longer there by Sunday.

Norfolk Southern has opened an assistance center in the village to gather information from affected residents.

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