WILLISTON, N.D. (AP) -- Federal and state investigators are at a Williston oil supply and logistics company where a massive fire erupted and disrupted road and air traffic.

Williams County Emergency Services spokeswoman Lorri Wilson said officials with the state fire marshal's office and the federal Environmental Protection Agency were at Red River Supply on early Wednesday.

She said it's uncertain when they'll determine the cause of the blaze that started midnight Monday and sent fireballs hundreds of feet into the air. The fire has since been extinguished.

No one was hurt but people within a half-mile area were urged to leave. Wilson said some did but most stayed.

State records show several flammable chemicals were on the property. The state Health Department and other agencies had monitored air quality in the area.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

A massive blaze that shot fireballs into the sky at an oil supply and logistics company in a western North Dakota oil patch hub has been extinguished, according to the town's fire department.

Most firefighters returned from the Williston facility belonging to Red River Supply late Tuesday, though one truck remained early Wednesday to monitor hot spots, said fire department shift captain Steven Kerzmann.

"The fire is out, but you've got some smoldering embers that might pop up here and there," he said.

The blaze, which started around midnight Monday, grew so intense that it sent fireballs hundreds of feet into the air. Kerzmann said early Wednesday that the department is not sure what caused the fire.

State records show several flammable chemicals were on the company's property. The company's website says it provides storage, blending and delivery of drilling fluids, among other things. It did not respond to a request for comment.

At a news conference about 15 hours after the blaze began, Williston Fire Chief Jason Catrambone said officials couldn't identify which specific chemicals had burned at the site.

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 monitoring air quality found nothing of concern Tuesday morning.

A large plume of smoke rose above Williston on Tuesday. It prompted the Federal Aviation Administration to issue a six-hour temporary flight restriction that lasted until 2 p.m. for Williston's Sloulin Field International Airport, airport manager Steven Kjergaard said.

Roads were closed Tuesday near the site, including a stretch of State Highway 1804. Local law enforcement officers cordoned off a half-mile area around the site, diverting traffic.