WILLISTON, N.D. — 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 Lorrie 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.