COLUMBUS, Wis. (AP) — A fire at a chemical plant that triggered explosions and forced the evacuation of about 140 homes was burning itself out Tuesday and residents were allowed to go home.

The blaze at a warehouse of the Columbus Chemical Industries plant erupted Monday night. Fire crews decided to withdraw and let it burn after explosions shook the building, slightly injuring three firefighters who were knocked off their feet.

The fire was still burning in a corner of the warehouse late Tuesday afternoon, and smoldering continued in the evening.

The U.S. Environmental Protection Agency determined that air and water outside the plant was safe, and all evacuees were allowed to go home.

It's not clear yet how the fire started, said Steve Quandt, the company's executive vice president. No workers were at the plant at the time.

State officials planned to investigate the cause of the fire on Wednesday, though Sheriff Todd Nehls said the fire didn't appear to be suspicious.

"This is purely what we do with a thing of this magnitude," Nehls said.

Resident Kim Buss, 44, said one explosion shook her house about a half block from the plant.

"When it exploded, it was like a black mushroom. Looked like fireworks," she said. "Then we saw all the fire trucks leave."

Quandt said officials monitored air quality and found nothing that exceeded federal air standards.

But Buss' boyfriend Jim Neuman, 43, said the air smelled like sulfur and that he, Buss and her son woke up with sore throats. "We got a blast of something," Neuman said.

Columbus Chemical Industries provides chemicals to high-tech and pharmaceutical companies, among others, according to its Web site.

Records show the plant was cited in 2005 for 10 safety violations, including eight labeled "serious," for issues including handling of flammable and hazardous materials and maintenance of safety equipment. Federal safety officials fined the plant about $4,300.

Occupational Safety & Health Administration spokesman Scott Allen said the company promptly corrected each violation and has had no problems since.

Quandt apologized to the community for the evacuations. "I would like to apologize to all the people we inconvenienced," he said at a Tuesday morning news conference.

Columbus is about 25 miles northeast of the state capital, Madison.