SPOONER, Wisconsin (AP) -- An explosion at a chemical plant in northwestern Wisconsin severely injured two workers and forced the evacuation of homes and businesses, authorities said.

 

The plant, owned by Cortec Corp., makes aerosols and paint solvents and has about eight employees.

 

''The bright red flames were 30 to 40 feet (9 to 12 meters) in the air and the smoke was pitch black,'' said Minong resident Jim Biros, who drove by about five minutes after the blast Tuesday. ''There were pieces of ash falling everywhere.''

 

The cause of the explosion was not known. It appeared no hazardous chemicals were involved in the fire that broke out at the time of the blast, Wisconsin Emergency Management spokeswoman Lori Getter said.

 

The cause of the explosion is ''really a mystery at this point in time,'' said Cortec vice president Anna Vignetti.

 

Homes and businesses within a half-mile (800 meters) of the plant were evacuated, the state patrol said. Residents were allowed to return Tuesday afternoon after tests showed the air was safe.

 

The two injured workers suffered what were believed to be first-or second-degree burns, Vignetti said. Both were reported in critical condition Tuesday night.

 

Another four employees inside at the time of the blast were not hurt.

 

The blast destroyed much of the building, the patrol said. The plant, about 100 miles (160 kilometers) northeast of Minneapolis-St. Paul, was shut down pending further investigation.

 

The plant was evacuated, and emergency workers set up a two-block perimeter around it, Getter said.

 

Nearby schools were keeping children inside after the explosion, Getter said. Spooner has a population of about 2,700.

 

St. Paul, Minnesota-based Cortec manufactures products for corrosion control.