Investigators have determined a stray spark from a nearby welding operation caused the fatal fire that took the lives of two Boston firefighters last week.

Police Commissioner William Evans said he was “confident’’ that the fire was unintentional, but that the Suffolk District Attorney’s office would have final say on whether anyone would be held legally responsible for the blaze.

High winds from the nearby Charles River caused the flames to spread quickly throughout the wooden structure, Fire Commissioner John Hasson said.

There were no sprinklers in the building.

Evans told reporters that he did not believe a fire detail or work permits were pulled for the nearby welding operation on Back Street, which abutted the property at 298 Beacon Street.


Hasson said the project, related to the repair of a nearby railing, should have required a formal fire permit and the presence of a fire extinguisher.

“This remains an active investigation in the capable hands of the Boston Fire Department, Boston Police Department, and the Suffolk District Attorney’s Office,’’ Boston Mayor Marty Walsh said in a statement. “I am confident they will pursue the appropriate course of action in this case, and they have my full support as they continue this investigation and evaluate their next steps.’’

Richard Paris, the president of Boston Fire Fighters Local 718 thanked the Boston Fire Investigation Unit, the Boston Police Department, and the Suffolk County District Attorney’s office for their help over the past week in a statement on Friday.

“The city of Boston did not just lose two of our best firefighters, but we buried two exemplary men this week,’’ Paris said in his statement. “We will await the result of the Suffolk County District Attorney Office investigation into the events that led to the blaze that killed our brothers Lt. Ed Walsh and FF Mike Kennedy. Boston Firefighters Local 718 and the Professional Firefighters of MA also support legislation that would lead to criminal charges if negligence is found to be a factor in the fire.’’


Oliver Realty Limited Partnership, the owners of 296 Beacon Street, released a statement, saying they are “deeply saddened by the tragic fire.’’

“Our hearts go out to the families of the firefighters who gave their lives to save others and protect property,’’ Oliver Realty Limited Partnership said in a statement. “The company recently contracted with a third party to install safety railings at 296 Beacon Street. It was the company’s understanding that these railings would be prefabricated offsite. We have cooperated fully with the investigators, providing them with all relevant information, and will continue to do so. The company also expects to launch its own review.’’