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WILLISTON, N.D. — A massive fire at an oil supply company was burning itself out Tuesday afternoon, 12 hours after it began in an industrial part of the western North Dakota oil patch town of Williston.

Authorities had not said as of Tuesday afternoon what caused the fire at Red River Supply, which Williams County Emergency Manager Mike Hallesy said broke out about midnight Monday. Company officials did not immediately respond to requests for comment and law enforcement wouldn't let anyone past the half-mile evacuation border.

Department of Health Air Quality Division director Terry O'Clair said local fire departments are letting the blaze burn itself out. He also said a National Guard hazmat unit is heading to the scene, which is in an area east of downtown filled with warehouses and businesses that service the oil industry.

A large cloud of gray smoke hovered over Williston on Tuesday morning. The North Dakota Department of Health and other agencies are monitoring air quality near the site for any potential threat, according to agency spokesman Tim Wiedrich. Williston Mayor Howard Klug said a hazmat unit that monitored air quality Tuesday morning found nothing of concern.