There were a number of POW camps which were never registered with the red cross. One in particular was Aikawa on Sado Island 500 prisoners were taken to work in the mine there in the early stages of the war, many died, the remaining 382 were entombed on the 2nd August 1945. The signal issued prematurely was to kill all prisoners
Here is the true story concerning the mass murder of 387 British and Allied prisoners of war by entombment. Taken from captain Godwin's War Criminals investigation office file. 125 M which the captain was ordered by general MackArthur to destroy in 1948.
Statement taken from Lt Yoshiro Tuda formerly second in command of the forced labour camp on the island of Sado, which was an unlisted prisoner of war camp.
Lt Yoshiro Tsuda, although evasive throughout his interrogation, answered most questions sufficiently helpful to enable the investigating officer to piece together the reasons and cause of the disappearance of 387 Allied prisoners of war, and including the date of their execution.
On the morning of the 2nd August 1945, I was ordered by major Masami Sadakichi the camp commandant, to detail the usual working parties at the nearby mine but with special instructions to ensure that every prisoner entered the mine. Usually fifty prisoners remained on top of the mine to empty the rakes of laden steel bins into nearby hoppers.
I pointed out this need to major Sadakichi, but he dismissed my concern with the comment that the mine was no longer viable and would be abandoned that day. Superior orders decreed that all prisoners of war were to be ordered to the deepest part of the mine, some 400 feet. Major Sadakichi further impressed on me that the guard detail were to carry out their duties in the normal manner, and not to alarm the prisoners.
I further advised major Sadakichi that a demolition detail had set concealed explosive charges inside the mine at depths of 100, 200, and 300 feet. This task had been carried out during the previous night. After the prisoners had been set hewing the ore from the marked areas . I was ordered to instruct Sgt major Mitsonobu Sakamoto the NCO in charge of the guards, to ensure their discreet out of the mine. The toiling prisoners were to be left to their obvious fate.
Between 8-45 and 9 am on the morning of the 2nd August 1945 All of the guards emerged from the mine tunnel whereupon a number of steel ore bins were pushed to the mines downward entrance and allowed to gather speed into its depths. At 9.10 am and with no further bins to dispose of , a signal was given to blow up the mine. I was watching from a distance of 100 yards and witnessed a rush of smoke and dust from the mines entrance. While waiting for the smoke and dust to clear, every available guard was set to work dismantling the steel narrow gauge track , and then carrying portions of it to the mines entrance. By 10-30am or thereabouts all traces of the steel track had been removed. From what I can recall the demolition detail then entered the mine to set more explosives just inside the mine entrance. It was while returning to the prison camp that I heard a very loud explosion. Looking back I saw an avalanche of rock and earth was completely covering where the mines entrance had been. Knowing that the mine had been collapsed in three separate places, I was certain that the prisoners were doomed.
Upon returning to the camp I immediately reported to major Sadakichi that the mine had been totally destroyed and all 387 POWs entombed in its depths.