Police said a door in the rear of the building at 24 Estey St. was found open and the padlock was missing when investigators arrived at the scene last week.
The rear door provides access to the building's basement, where the fire appears to have started, police said Thursday.
No suspects have been arrested in connection with the crime, although police said several juveniles were seen in the area before the fire began.
Brattleboro Fire Chief David Emery said investigators with the Brattleboro Police Department and Vermont State Police spent the last week digging through the rubble in the home's basement and interviewing former residents.
The March 18 fire broke out shortly before 6 p.m. and quickly spread through the structure to each of its four floors. Residents escaped the building unhurt after fire alarms in the hallways went off.
The building _ which fire officials believe may have once been home for Estey Organ Co. workers in the 1900s _ is owned by Dale Everett of Florida, the brother of a local real estate broker.
The displaced residents spent several days at the Super 8 Motel in Brattleboro, courtesy of the Red Cross, Emery said. Several of the residents are still staying at the motel while others have found new apartments or are staying with family, he added.
The Brattleboro Lions Club has begun fund raising for the 30 tenants displaced by the fire. An account at Brattleboro Savings & Loan has more than $1,100 in it, according to a bank spokesperson.
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