(CNN) -- The blaze that killed two Boston firefighters last week was unintentionally caused by a welding operation, Boston fire officials said Friday.
But prosecutors have yet to determine whether anyone was criminally culpable, Boston Police Commissioner William Evans told reporters. The welders did not have a required permit.
The nine-alarm fire occurred on March 26 at a brick brownstone in Boston's Back Bay neighborhood.
Lt. Edward Walsh, 43, and firefighter Michael Kennedy, 33, were killed. Thirteen other firefighters were injured.
Fire Commissioner John Hasson told reporters that the blaze was caused by sparks from welding in the rear of the building. The sparks probably festered in the wooden structure, with flames later fed by strong winds off the Charles River.
"We're confident this was an unintentional death at this time," Evans said. "Whether anyone should be held culpable for this event, as the investigation goes on, that will be a determination made by investigators in the District Attorney's Office."
Ed Zabin, chief of the homicide unit for the Suffolk County District Attorney's Office, said investigators would try to determine whether anyone was criminally responsible. He said welders were installing an iron hand rail behind the building.
Evans and Hasson said that no fire detail was present at the job site and that the welders did not have a required permit.
"In most cases," Hasson said, "a fire detail is required."
Walsh had worked with the Boston Fire Department for close to 10 years, and Kennedy had 6½ years on the job.
Walsh was married with three children, all under the age of 10. Kennedy was a veteran of the U.S. Marine Corps.
Walsh and Kennedy were trapped soon after entering the building. They were later found in the basement, where the fire appeared to have started.
Fueled by strong winds, flames quickly engulfed the four-story building. An explosion at one point threw a number of firefighters down stairs.
CNN's Jason Hanna contributed to this report.