ST. LOUIS - St. Louis Fire Chief Dennis Jenkerson says he believes a boiler explosion that killed three people and injured four others, two critically, in two different businesses was a commercial accident.

Jenkerson says two of the four survivors of the blast shortly before 8 a.m. Monday remain in critical condition. He says one of them was undergoing surgery.

Jenkerson says the boiler exploded at the Loy-Lange Box Co. in south St. Louis, killing one person there. The chief says much of that equipment flew about 500 feet (150 meters) across the street onto a laundry business, killing two people there when it came through the roof. The equipment is described as being the size of a van

Federal workplace safety officials are investigating.