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ROCHESTER, Ind. (AP) _ An explosion destroyed a fire extinguisher warehouse containing dangerous chemicals, killing four people and critically injuring a fifth person who was covered in flames as he tried to escape the inferno.

Two other people inside the concrete building escaped unharmed after the explosion on Wednesday blew an office door shut, protecting them from flying debris, authorities said.

``It was a small building to begin with, and it was pretty well destroyed,'' Mayor Phil Thompson said. ``The whole sky was filled with smoke up there, so it must have been pretty big explosion.''

Columns of smoke filled the air, and the area had to be closed off because of the smoke, said building owner James Eikenberry, who lives about 50 yards away. The blast happened at the Pyro Design and Development warehouse, about 30 miles from South Bend.

The victims were believed to be two women and two men, Fulton County Coroner Jon D. Good said. Another man was hospitalized with severe burns.

Jerry Burkett, a retired volunteer firefighter, said he and his wife heard the explosion at their home about a half-mile away and headed for the warehouse to see if they could help. He said they found the injured man walking from the building.

``He was on fire,'' Burkett said. ``All you could see was his eyes.''

Burkett's wife wrapped the man in an old blanket and extinguished the flames. The couple drove him to Woodlawn Hospital, and he was then flown by helicopter to a hospital in Fort Wayne, authorities said.

The chemicals kept in the warehouse were categorized as extremely dangerous and included chromium nitrate and sodium bicarbonate, among others.

Agents from the U.S. Bureau of Alcohol, Tobacco and Firearms were investigating the cause of the explosion, spokesman Pat Berarbucci said.