NHK has learned that security guards at Shuri Castle in Okinawa Prefecture, southern Japan, first detected smoke in the northern side of the main hall in Thursday's fire.
The security guards reportedly told police that they found smoke had filled a stairway leading to the northern part of the main building.
They are also quoted as telling police that the smoke filled the area, making it difficult to breathe.
Police say security personnel entered the main hall from their office at the gate just across from the building after being alerted by the security system.
Police suspect the fire started in the northern section as the smoke reached a corridor leading to that area soon after a sensor sounded an alert. They are now trying to determine the cause of the fire.