Police in Okinawa believe the fire which destroyed Shuri Castle is likely to have started from the main hall. One of the first witnesses saw smoke coming out the first floor of the building.
The blaze engulfed the castle in Naha City as it raged for 11 hours on Thursday. Most of the structures, including the wooden 3-story main hall, were burned to the ground.
Investigators told NHK that one of the guards who responded to the security alarm witnessed heavy smoke billowing out of windows on the first floor of the hall.
They also say surveillance cameras inside the building captured a flash of light which is believed to be the start of the fire.
Police say it will take some more days to complete the on-site investigation as a large amount of burned debris is hampering the work.