An explosion that may have started in a large propane tank flattened an industrial warehouse near downtown Milwaukee on Wednesday, killing at least three people and injuring 37 others, authorities said.

The fiery blast at the Falk Corp. factory flipped cars, hurled debris into the air and forced the evacuation of dozens of workers at the plant, which makes large industrial gears and couplings. Burning rubble was spread over several blocks.

"It sounded like when I was in Vietnam, incoming mortar rounds," said worker David L. Mays, 61, who said the blast knocked him down and turned over his car. "But I'm OK," he said as a tear rolled down his cheek.

Fire officials had not determined the cause of the blast, but Mays said the company began to evacuate workers after a leak occurred in one of six large propane tanks. He said he and others were outside when one of the tanks blew up.

As of midafternoon, hours after the blast, authorities had yet to account for all of the plant's employees.

Falk employs about 700 people in Milwaukee. Company spokeswoman Linda Mayer said she did not know how many people were in the plant at the time.

Machinist Jim McGraw, 53, said a co-worker warned him about a minute before the blast that he could smell gas, but McGraw did not think much about it because the building normally smells of solvents and other chemicals. Then the explosion knocked him down.

"I didn't know what it was at first, just a loud bang," McGraw said.

He looked around and saw co-workers with blood on their faces. He grabbed a rag and tied it around his arm, which had a puncture wound. McGraw was taken to a hospital, where doctors pulled a piece of a glass from his skull.

As many as 120 firefighters and paramedics responded to the fire, along with hazardous materials crews. The flames were largely extinguished within three hours.

The explosion destroyed Falk's warehouse and numerous cars and damaged several other buildings in the complex, fire Lt. Brian O'Connor said, adding that the blast could be felt at a firehouse about a mile and a half away.

Mayor Tom Barrett said the company had a clean safety record. The factory was last inspected in September and "any violations were corrected on the spot," he said.

The Occupational Safety and Health Administration sent a team to investigate.

Construction crews working on buildings in the surrounding area were sent home because of smoke from the fire.