Officials say sparks from a welding operation next to a Boston brownstone caused the blaze that killed two firefighters last week.
Boston Police Commissioner William Evans says officials are confident the fire was not intentional. They have not decided whether anyone will face charges.
Fire Commissioner John Hasson says it appears the welding sparks got into the building. Officials would not say how many welders were working. They said it does not appear anyone got a permit for the work.
Firefighter Michael Kennedy and Lt. Edward Walsh died after they were trapped in the basement of the Beacon Street apartment building 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.