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The Boston Fire Department’s fire prevention division’s office is closed for the rest of the day after being evacuated when an envelope containing a powdery substance was opened early this afternoon, the department said. Firefighters responded to the fourth-floor office at 1010 Massachusetts Ave. just after 1 p.m., Boston fire and Boston police said. The area where the envelope was opened was isolated and the floor evacuated as crews shut off the building’s air system, department spokesman Steve MacDonald said in messages posted on Twitter.