More than half a day after an industrial fire broke out in the North Dakota oil patch town of Williston, authorities still were unable to say what caused the massive blaze.
The fire, which started around midnight Monday and continued to burn late Tuesday afternoon, grew so intense it sent fireballs hundreds of feet into the air at a facility belonging to oil supply company Red River Supply.
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. He said firefighters were monitoring the blaze while letting it burn itself out.
"We are not overly concerned at this time with the current situation," the chief said, adding that the fire was "pretty well contained," though flare-ups were likely to continue.
He said firefighters were limiting the amount of water they were putting on the fire due to potentially volatile reactions some chemicals at the facility have with water and to minimize the further contamination.
The North Dakota Department of Health was monitoring air quality.
State hazardous-chemical records show several flammable chemicals were on Red River Supply's property. The company did not immediately respond to interview requests but issued a brief statement that thanked emergency responders and said the fire's cause had yet to be identified.
A large plume of smoke that rose above Williston early Tuesday was a primary safety concern for the town's residents. Williams County Emergency Manager Mike Hallesy said it's not clear what particulates were in the plume of smoke, but that the cloud did not blow toward the city.
Local law enforcement officers cordoned off a half-mile area around the site, diverting traffic. Officials said there were very few residences in the mostly industrial land that fell within that area.
"We did encourage people to evacuate if they were concerned about the smoke," Hallesy said.
The smoke plume prompted the Federal Aviation Administration to issue a six-hour temporary flight restriction that lasted until 2 p.m. Tuesday for Williston's Sloulin Field International Airport, airport manager Steven Kjergaard said, and a flight restriction remained directly to the southeast of town.
Officials were grateful that the plume did not get pushed toward the city.
"The wind helped us today," Hallesy said. "For once, the weather was on our side."
Hallesy said an Environmental Protection Agency official was on the way to Williston from Denver and would further investigate the particulates in the plume of smoke.
Around 1 a.m. Tuesday, fireballs could be seen at the site. They were accompanied by the pop of explosions as thick black smoke billowed above towering flames. Catrambone said the explosions complicated firefighting efforts early on.
Red River Supply's site has rail spurs for loading and unloading liquids, such as crude oil, and dry goods, according to its website. The company's website also says it provides storage, blending and delivery of drilling fluids. State hazardous chemical records shows a number of flammable chemicals were stored on the company's property.
Emergency responders were expected to remain on the scene for the next day or two, Hallesy said.