At least three people are dead and dozens others injured in an explosion and fire at a manufacturer in Milwaukee's industrial valley.

Mayor Tom Barrett also says an unconfirmed number of people are missing.

Fire Chief William Wentlandt says the search of the rubble and debris from a flattened warehouse continues at the Falk Corporation complex. Aerial video shows at least two dozen vehicles were strewn around the debris field of about two and a-half blocks.

Other buildings at the Falk complex were damaged by the blast.

Falk employee David Mays says the company began to evacuate workers after a leak occurred in one of six large propane tanks behind the building called the annex. He and others were outside when the explosion happened. The force of the blast knocked Mays to the ground.

Barrett said at an afternoon news conference that 37 injured people had been transported to area hospitals. They included one man critically injured by a blast to his chest.

Police Chief Nan Hegerty says a preliminary investigation shows the explosion and fire was an accident. Hegerty says there's no indication it's related to any criminal activity.

The Fire Chief says 120 firefighters and emergency workers responded to the explosion and fire, which began around eight a.m.

The Falk Corporation is headquartered in Milwaukee and manufactures power transmission equipment.