Investigators from the Rochester Fire Department, the Rochester Police Department and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives spent 12 hours gathering evidence in a still-smoldering duplex at 82 Grape St. Fire Chief John Caufield said he could not not say where in the house the fire began and whether it was started accidentally or by an act of arson. “We have theories, but need to further investigate to determine what happened,” Caufield said. “We don’t want to speculate.”