Officials announced on Friday that the cause of the Boston fire in the Back Bay neighborhood last week was the result of sparks from welders.

The welders were working on the neighboring building and sparks were able to get under the brownstone building's clapboards, where it was allowed to grow thanks to winds, reports ABC News. It isn't known how long the fire was smoldering before it broke out into a large blaze because the welders didn't report anything.

Boston Police Commissioner William Evans noted that the welders likely knew about the fire, but failed to let anyone know. Massachusetts Sen. Ken Donnelly said, "It is an absolute outrage. There needs to be criminal charges in this matter."

The welding company, which has not been revealed, may face criminal charges relating to the fire which killed two firemen, but it would be difficult to actually bring charges against the company because the state doesn't have a criminally negligent homicide statute.

The Los Angeles Times reports that no permit had been filed for welding work on the neighboring building, but one isn't required by law.

The two firefighters who perished, Lt. Ed Walsh and Michael Kennedy, were honored at funerals that were held on Wednesday and Thursday. Kennedy was also a U.S. Marine combat veteran.