Sparks sprayed by welders working on an iron railing caused the fire that tore through a Boston brownstone, killing two firefighters and injuring 13 other people, officials said Friday.

The funeral procession for Boston firefighter Michael R. Kennedy proceeds through saluting firefighters as it arrives at Holy Name Church in Boston, Thursday, April 3, 2014. Kennedy and Boston Fire Lt. Edward J. Walsh were killed Wednesday, March 26, 2014 when they were trapped in the basement of a burning brownstone during a nine-alarm blaze.(AP Photo/Stephan Savoia)
The funeral procession for Boston firefighter Michael R. Kennedy proceeds through saluting firefighters as it arrives at Holy Name Church in Boston, Thursday, April 3, 2014. Kennedy and Boston Fire Lt. Edward J. Walsh were killed Wednesday, March 26, 2014 when they were trapped in the basement of a burning brownstone during a nine-alarm blaze.(AP Photo/Stephan Savoia)
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Boston Fire Commissioner John Hasson said it did not appear anyone had obtained a permit for work on the railing behind a building attached to the apartment building that burned.

The sparks somehow got into the building and the fire, fanned by strong winds off the Charles River, spread quickly. One senior fire official said that in 30 years he had never seen a fire spread so fast and cause so much damage.

"Welding is that dangerous," Hasson said. "That's why we require a permit."

Investigators have not yet decided whether anyone will face charges in the fire. Under Massachusetts law, the only way someone can be charged with negligent homicide is in some car accidents. Police Commissioner William Evans did say officials are confident the fire was unintentional.

Firefighter Michael Kennedy and Lt. Edward Walsh died after they were trapped in the basement of the building in the Back Bay neighborhood on March 26.

Both men were laid to rest this week in emotional funeral ceremonies attended by thousands of firefighters from across the state and nation.

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