Sparks from welding work being done in the rear of 296 Beacon St. flew toward a neighboring building, ignited it and eventually claimed the lives of two Boston firefighters, authorities said.
The cause of the Back Bay fire at 298 Beacon St. last week was sparks from that welding work to install or repair an iron handrail. Authorities announced the cause of the 9-alarm fire just a day after the last of the two funerals were held for Lt. Edward Walsh and Firefighter Michael Kennedy.
Boston Police Commissioner William Evans said there was no fire detail or a permit pulled for the welding work being done at the rear of the building. A permit is required by law and in most cases a fire detail is required, said Interim Boston Fire Department Commissioner John Hasson.
Hasson said once a permit is pulled a fire official will go to the work site to determine the potential hazards and decide whether or not a fire detail is necessary.
Authorities said the investigation is ongoing. They did not identify the welding company.
"The nature, extent and circumstances of that work remain under intense scrutiny by investigators and prosecutors assigned to this case," said Ed Zabin, chief of homicide for the Suffolk district attorney's office.
Walsh and Kennedy died March 26 after they became trapped in the basement of the Back Bay brownstone. At the time, fire officials said they believe the fire started in the basement of the building and spread quickly throughout the home because of strong winds that day.
Mayor Marty Walsh said he was encouraged by the progress of investigators.
"This remains an active investigation in the capable hands of the Boston Fire Department, Boston Police Department and the Suffolk district attorney's office," 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."