The first limited research in the Jasenovac concentration camp, was conducted on May 11, 1945 bythe District Commission for the Crimes of Occupiers and their Assistants from Nova Gradiška.

The State Commission of the Republic of Croatia for the Investigation of War Crimes of Occupiers and their Assistants determined on May 18, 1945 the number killed in Jasenovac ranged from about 500,000 to 600,000 but that the exact number of victims in the Jasenovac concentration camp could never be accurately determined.

The first panoramic aerial film of the Jasenovac concentration camp was made in 1945. Footage covered the northern part of the camp. However, film footage of Donja Gradina has not been found.

The Yugoslav Air Force recorded the flow of the Sava River in 1946; however, the negatives that covered most of Donja Gradina also have not been found.

Nikola Nikolić, a former inmate at Jasenovac, and his son Sergij did research in 1947 in Donja Gradina and found 248 clearly visible mass graves whose dimensions were 60-80 m long, 6 m wide, and 4 m deep. On the right bank of the Sava River, Nikolić discovered two wagons full of ashes, burned skulls and vertebrae that had been thrown away there after the corpses were burned at Donja Gradina.

The first photogrammetric records were made by The Institute of Photogrammetry in Belgrade in 1957.

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