An explosion ripped through a truck manufacturing plant in the small southern Minnesota town of Dodge Center late Wednesday morning, injuring at least six people, authorities said.

Two workers at McNeilus Truck and Manufacturing were flown to Regions Hospital with severe burn injuries and two were taken by ambulance to St. Mary's Hospital in Rochester, where they were being treated for "serious injuries," said Dodge County Sheriff Scott Rose.

Two employees were being treated at the scene for minor injuries, he added.

Investigators were trying to determine what caused the blast that was reported at 10:32 a.m. inside the north complex of the sprawling plant that produces cement and garbage trucks powered by compressed natural gas.

The explosion occurred in a "paint booth" where there was work going on, Rose said. But it was not clear if paint fumes or natural gas may have ignited, the sheriff said.

The blast was strong enough to cause extensive damage to the long and narrow complex, which features several production lines. Doors were blown off and there was lots of smoke, Rose said.

Police, firefighters and paramedics from multiple agencies rushed to the scene. The road in front of the plant, County Road 34, was blocked off for about three hours. It reopened around 1:30 p.m.

More than 1,000 people work at McNeilus Truck and Manufacturing, but many of them were off site attending training Wednesday.

"We are fortunate from an employee standpoint that we didn't have more injuries due to this incident," Rose said.

Rose said all employees who were on the job at the time have been accounted for and only the six people were hurt.

"If you have not been contacted about a family member, they are OK or were not involved in the incident," Rose said.

All production employees were sent home for the day. It was not clear when work at one of the largest employers in the county would resume.

Rose said the company had safety plans in place.

"They run a top-notch company," he said. "We don't see many calls here."