Spilled molten steel caused a fire and explosion Monday at a Michigan steel plant, injuring 12 people, officials said.

A transport vehicle that can carry up to 400 tons of molten steel was backing into a building at the sprawling River Rouge (search) industrial complex when some of its high-temperature cargo sloshed out, said William Hornberger, a spokesman for Severstal (search), the Russian company that owns the plant.

The molten steel hit two of the vehicle's 8-foot tires, setting them afire and causing them to explode minutes later while Dearborn firefighters were battling the flames, Hornberger said. The explosion blew out part of the building's steel sheet siding and caused electrical damage, he said.

Three were hospitalized in stable condition Monday afternoon; the other injured people were treated and released at hospitals.

John Brennan of the Michigan Occupational Safety and Health Administration (search) said a compliance officer was at the plant Monday to investigate the accident. An investigation could take a few weeks, he said.

Hornberger said the plant continued to operate after the accident, although at a reduced level of activity to accommodate the investigation and cleanup.

The River Rouge facility, built by Ford Motor Co., once was the world's largest industrial complex. Ford still has operations there.

In 1999, a Ford boiler exploded at the complex during routine maintenance, killing six employees and injuring 14 others. State regulators later found 15 workplace safety violations.

In 2001, a furnace blew and 2,000-degree molten steel spewed out, causing minor injuries to two workers.