Investigators say they know the cause of Friday’s fire that struck an historic apartment-house in Denison, leaving a resident and a firefighter with minor injuries. The state fire marshal’s office says a short in a lamp cord in a second-story apartment is blamed for sparking the blaze. The building had major fire damage on the upper level with smoke and water damage on the main floor. Local historians say the home, built in the 1870s, may be the oldest structure in Denison.