Police Commissioner William Evans said the fire is still under investigation, and the Suffolk County district attorney will make any decision on criminal charges. But Evans said he is "confident" arson was not involved.
The fire in a building on Beacon Street in the Back Bay neighborhood spread quickly because of high winds March 26. Two firefighters, Lt. Edward Walsh and Michael Kennedy, were trapped in the basement.
At Friday's news conference, Fire Commissioner John Hasson said the welding work to a railing near the building required a permit and a fire extinguisher.
Mayor Marty Walsh, in a statement, said the police and fire departments and the district attorney will continue the investigation.
“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,” Walsh said.
Edward Walsh and Kennedy were buried this week with thousands of firefighters from around the country present.