"I (opened) the door and we (saw) dust flying ... stuff flying everywhere", Rocky Pruneau, who works in the building across the street from Loy-Lange Box Co., told the Post-Dispatch.
Two people are in critical condition and two others are seriously injured.
The other two buildings were damaged by debris from the first building, authorities said.
Federal workplace safety regulators say a St. Louis company whose boiler exploded causing at least three deaths Monday has paid fines for workplace violations three times since 2014.
One person died when the van-sized boiler exploded at Loy-Lange Box Co. on Russell Boulevard. While at least initially believing the explosion was accidental, Jenkerson said investigators will seek out and review the boiler's inspection and maintenance records.
The scene is under control and no foul play is suspect, according to fire officials.
Back in February of 2014, Lange Box paid out $2,450 for defective energy control procedures, for example, not properly training workers to ensure machinery was turned off and powered down.
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In November of 2014, an inspection revealed defective equipment, and Lange Box paid just over $6,500 in fines.
Investigators from the US Department of Labor's Occupational Safety and Health Administration were working on the scene.
None of the victims' names have been released as of early Monday afternoon.
A pipe or pole also flew from the Loy-Lange Box Co. building across a parking lot and landed vertically on the windshield of a man's pickup. The agency's online records show that Loy-Lange was cited past year for an unspecified "general requirement" and paid a $3,741 fine, half of what OSHA initially assessed. Allen couldn't say if that violation had anything to do with the boiler.
"We are grateful to the firefighters and other emergency responders who have acted heroically in response to this tragic event", Spence said. FOX 2 will keep you updated as details become available.
Russell & S. Broadway - Reported boiler explosion affecting three bldgs w/debris.