An explosive fire in a Buffalo costume shop that killed a firefighter who was trapped has been ruled accidental and no criminal charges will be filed, a prosecutor said Thursday.

An investigation found that the March 1 fire began when a blowtorch used on the brick exterior of the three-story building ignited bags of clothing inside, Erie County District Attorney John Flynn said at a news conference.

The fire sparked at least two explosions that knocked firefighters who were outside the building off their feet, and sent smoke and debris billowing into the downtown street. Fire Commissioner William Renaldo described an explosion caught on video as a backdraft, which he said occurs when oxygen is "sucked into the building and then blown back."

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It was after the first explosion and an evacuation order that firefighters determined firefighter Jason Arno, who had issued a mayday call, was missing. Flynn said firefighters went back in and found him trapped by a metal clothing rack but could not free him before a second explosion forced them back out of the building.

"A clothing rack must have fallen on top of him," Flynn said, "and his foot and leg got tied in to the metal rod on a clothing rack and he was stuck and they couldn’t get him out."

His body was recovered about 3 1/2 hours after the fire was reported. He died of smoke inhalation and burns, Flynn said.

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Arno, 37, had been married over the summer and had a 3-year-old daughter. He had been with the fire department for three years.

Investigators determined that the contractors using the blowtorch had used a metal shovel to direct the flame away from a plywood door but that a spark likely traveled underneath the door and ignited clothing, Flynn said. In ruling out criminal charges, the prosecutor said there was no evidence the workers had disregarded the potential danger of using the blowtorch to melt snow and ice from the sidewalk and building before beginning masonry work.

"It's stupid but not criminal," he said.

The building, empty except for the costume shop, had been purchased four months earlier by former U.S. Rep. Chris Jacobs.