In April 1945, Ustashas commenced the final liquidation of the camps. This implied killing of all remaining prisoners, destroying of camp archives and burning and razing of camp buildings to the ground. However, what needed to be done first was to conceal the traces of crimes committed in Jasenovac. An operation aimed at concealing the traces of crimes thus started and lasted from 06 until 20 April 1945. Prisoners dug out graves in Donja Gradina taking out mortal remains of Jasenovac victims and burning them. They would put coke under the fire pits made of railway rails. Corpses were put on rails; oil was poured on them and they were burnt. The ashes of burnt corpses were mainly thrown into the Sava River or buried in earth. A new group of prisoners went to Gradina every day because the group from the previous day had been killed.