Melvin Kuster retired after 41 years working for Falk Corp., the century-old factory that also once employed his father. Now his only child has died there.

Daniel Kuster, 35, and two other workers were killed Wednesday when an explosion and fire leveled the industrial warehouse and rocked the near-downtown area. Forty-six other workers were injured, including one with life-threatening injuries, Mayor Tom Barrett said.

"You shouldn't have to bury your kids," Kuster's father, Melvin, said, crying, Wednesday night.

Police Chief Nan Hegerty said investigators were still trying to determine the exact cause of the blast, which shook the surrounding area, overturned workers' cars in a parking lot and broke windows of nearby businesses and apartment buildings.

"There was a propane leak. There were employees of Falk who were investigating the leak," she said. "They began to evacuate the building, and that's when the explosion occurred. Had they not been evacuating the employees that were in that building, the death toll would have been much higher."

Hegerty said the explosion seemed to be an accident but police opened a criminal investigation, which was normal procedure. Federal agents also were investigating.

Barrett said the company has a clean safety record with the city. The last inspection was in September, "and any violations were corrected on the spot," he said.

Bob Hitt, CEO of Falk's parent company Rexnord Corp., said employees would be paid while the firm assesses the situation at the 61-acre complex.

The company held a private meeting with employees Wednesday evening to offer grief counseling and information.

Larry Thibault, 59, a warranty administrator at Falk, attended the meeting and said the company was optimistic about rebuilding, although officials didn't give a timeline for when people would return to work.

"There was an absolute commitment to bring the company back to full production as soon as they can," he said.

Police interviewed more than 500 people and firefighters searched through wreckage for hours before accounting for the more than 700 workers from the plant.

The factory, which is not far from the Milwaukee Brewers' home of Miller Park, is in the Menomonee Valley, home to several trucking companies and industrial supply firms.

Dozens of workers with injuries including broken bones, head wounds and cuts were treated at hospitals throughout Milwaukee.

The three men who died had years of experience at the plant.

Daniel Kuster was an only child, never married and had no children, said his aunt, Vicki Izydor, 36. She said he was a fork lift driver and had been with Falk for 11 years.

"I know he was very close to his parents," she said. "I couldn't even imagine to be honest what it feels like to lose your only child, and in such a tragic way."

Also among the dead were 38-year-old Curtis J. Lane, of Oconomowoc, and Thomas M. Letendre, 49, of Milwaukee.

Curtis Lane's father-in-law, William Borgiasz, said Lane was the father of two young children and his wife, Tina, operates a day care center.

"It is a big tragedy," Borgiasz said. "It is really hard to deal with. All we are going to tell them is, 'Daddy ain't coming home.'"