The U.S. Chemical Safety Board (CSB) has dispatched a four-person investigative team from to the scene of yesterday’s explosions and fire at a Husky Energy oil refinery in Superior, Wisconsin. Eleven people were initially reported injured in the blast, which occurred as workers were shutting the refinery down in preparation for a five-week turnaround. The number of injured was later adjusted to six, with one person in serious condition. There were no reports of fatalities, and all workers at the facility were accounted for.

A mandatory evacuation order was issued for Superior residents living within a three mile radius of the refinery and ten miles south of it due to the heavy smoke. More than 30,000 people were affected.

The fire raged for eight hours before being successfully extinguished by firefighters.

The first blast occurred at approximately 10 a.m., the second at 12:30 p.m. and then numerous explosions were heard throughout the afternoon. Fires followed several of the explosions.

According to news reports, authorities said a tank of crude oil or asphalt exploded, although it is unclear at this time what caused that to happen.

The CSB and OSHA will both investigate the incident.

The refinery processes around 50,000 barrels per day and has a storage capacity of 3.6 million barrels. It makes gasoline, asphalt and other products. The Duluth News Tribune reports only one enforcement action against the refinery by OSHA. It’s previous owners were fined $21,000 in 2015 for emergency response and flammable liquids violations, which were resolved that year.