Updated

A tanker truck exploded Tuesday afternoon in one of the busiest parts of this northwestern Iowa town, forcing the evacuation of about 2,000 people — about a third of the town's population.

One person was hospitalized with undisclosed injuries, officials said. The cause of the blast was unclear.

"We know that we have a trailer transport carrying some sort of flammable liquid adjacent to rail cars," said David Krahling, spokesman for the Sioux Center Fire Department. "I'm not exactly sure how full or empty any of those vessels were."

A flammable substance exploded at the Farmers Co-Op Society as it was being unloaded from a tanker into a rail car, and firefighters were called, officials said.

Authorities ordered an evacuation within a half-mile of the site, just west of the business district.

Lenore Prins, who lives one block from the co-op, said she could hear explosions.

"First it was one, and then probably three, four, five more," she said. "I looked out the window and there was a lot of black smoke and fire."

Emergency crews moved some residents to a recreation center, said Jerry Holtrop, chief deputy for the Sioux County sheriff's office. Crews extinguished the flames by 9 p.m. but had not lifted the evacuation order.

The approximately 100 people who work at the co-op had been accounted for, officials said.

The fire was contained to a small area, but the threat of further explosions kept firefighters on high alert much of the night, Krahling said.