If the military has established a date of death they must have looked into it so, there should be a detailed record of it. If he died in the hands of the enemy it would not have been difficult to find inmates who could attest to his death because the war ended only about 30 days later. Also, it is possible that the US Army had liberated Malthausen before he died so there could be a first hand account of the event. It is standard procedure in the US military to try to account for all missing and dead, with specific details, to the greatest degree possible. They go to great lengths. They are still finding remains from WWII in the jungles of the South Pacific.