Updated

Sparks from a welding project for which no one had obtained a permit caused the wind-driven fire that tore through a Boston brownstone last week, killing two firefighters and injuring 13, officials said Friday.

Police Commissioner William Evans said investigators are confident the fire in the city's Back Bay neighborhood was unintentional, but they have not decided whether anyone will face charges.

Boston Fire Commissioner John Hasson said it did not appear anyone had obtained a permit for the work on an iron railing in back of an apartment building next to the one that burned. The sparks somehow got into the building and the fire, fanned by strong winds, 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.

Officials declined to discuss specifics of the case, including the scope of the welding work, what firm the welders worked for, whether they called 911 to report the fire or whether they are cooperating with investigators.

Permits are required for all welding projects in Massachusetts and most also must have someone from the fire department watching to make sure they are being done safely. Hasson said no such fire detail was assigned to the project at the Beacon Street building.

Edward Zabin, a Suffolk County prosecutor, said human error in fatal fires does not always lead to criminal charges. He noted that Massachusetts, unlike many states, does not have a negligent homicide statute; the only way someone can be charged with that type of crime is in certain motor vehicle accidents.

Firefighter Michael Kennedy, 33, and Lt. Edward Walsh, 43, died after they were trapped in the basement of the burning building. Both were laid to rest this week in emotional funerals attended by thousands of firefighters from across the state and nation.

"It's been a tough week for the firefighters," Evans said.