Boston police, firefighters, and emergency medical workers responded to an office building in the Back Bay Friday morning after a man fell down an elevator shaft, authorities said. The man was an elevator operator working in the building at 31 St. James Ave. Emergency workers found him dead at the bottom of the shaft, said Boston Fire Department spokesman Steve MacDonald. The man will not be identified until autopsy results are released, police said. Investigators from the Occupational Safety and Health Administration and Boston police are trying to determine the cause, an OSHA spokesman said. Police said the preliminary investigation indicates an accidental death.