The blast happened as firefighters and six employees at the Isotec plant tried to contain the leak. No one else was in the plant at the time.

About 2,000 people living within a mile of the plant in the Dayton suburb were evacuated. Other chemicals were stored in the area where the explosion happened, including a large tank of carbon monoxide, according to officials.

One worker was treated for a cut on the head. The employee was standing near the plant entrance when the blast knocked him into a truck and chunks of concrete and metal fell from the sky, according to police. "I've never been this scared in my life," the injured worker told AP.

The plant is owned by Sigma-Aldrich Corp. in St. Louis, which supplies chemicals to research laboratories. A spill of nitric oxide at the plant in 1998 forced an evacuation of a golf course and about 50 residents, according to AP.