3 dead after boiler explosion in St. Louis The explosion sent a piece of equipment into the air and into another building.

 -- Three people died and several others suffered serious injuries after a boiler exploded at a building in St. Louis, sending a piece of equipment flying into the air and through the roof of another building, fire officials said.

The boiler explosion at the Loy-Lange Box Co. left one person dead in that building, authorities said at a news conference this morning.

After the explosion, a piece of the boiler was tossed into the air and pierced the roof of a nearby building, the Faultless Healthcare Linen plant, killing two people, officials said.

The mayor's director of communications described the piece of the boiler as "van-sized."

Two people were in critical condition, and two others suffered non-life-threatening injuries, the fire department said.

Mark Spence, the COO of Faultless Healthcare Linen, confirmed that two Faultless Healthcare Linen employees were killed from debris that came into the plant after the explosion.

"One other Faultless Healthcare Linen employee was involved and the extent of that person’s injuries has not been confirmed," Spence said in a statement. "We immediately will be giving what practical help we can to our employees and their families. We are grateful to the firefighters and other emergency responders who have acted heroically in response to this tragic event."

Spence said this location has 106 employees and that it could not yet be confirmed how many were in the building at the time of the incident.

The U.S. Department of Labor's Occupational Safety and Health Administration (OSHA) has opened an investigation because the incident involved fatalities, an OSHA spokesman told ABC News. Health and Safety Compliance Officers are at scene interviewing witnesses and employers to see whether any potential safety violations may have led to the incident, the spokesman said.

ABC News' Devin Villacis and Jason Volack contributed to this report.