A Beacon Street fire that killed two Boston firefighters last week was accidental and sparked by welders working at the rear of a nearby building, Boston Fire Commissioner John Hasson said Friday.

BOSTON -- The fire that killed two firefighters as it burned through a brownstone in Boston’s Back Bay neighborhood was caused by welders working at the back of a neighboring building, the Boston Fire Department said Friday.

Welders were working on an iron railing behind 296 Beacon St., Boston Fire Commissioner John Hasson said at a news conference. Sparks from the welding got under the shingles, “festered there for a while,” and then were fed by the wind blowing off the Charles River, he said. The two firefighters were trapped in the basement of 298 Beacon St. as the fire raged out of control.

There was no permit on that building for welding work to be done, Hasson said, although one is required by law.

Firefighters from across the country have arrived in Boston this week to attend funeral services for Lt. Ed Walsh and firefighter Michael Kennedy, who were killed in a 9-alarm fire that broke out just before 3 p.m. on March 26. Well-wishers have deposited hundreds of bouquets of flowers and other memorials at the firehouse that houses their companies on Boylston Street, in Boston’s Back Bay.

The firehouse is only a few blocks from the spots on Boylston Street where bombs went off at the finish line of the Boston Marathon on April 15 last year. This year's marathon will take place on April 21 on the same street.

The funeral for Kennedy, 33, a U.S. Marine combat veteran, was held in Boston’s Roxbury neighborhood on Thursday; the funeral for Walsh, 43, was held Wednesday in Watertown, Mass.

“In times of tragedy, of course we ask how something so terrible and senseless could happen,” Mayor Martin Walsh said in a statement.  “But I am encouraged that investigators involved in this case have made significant progress identifying the origin and cause of the fire.”

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