After the April war of 1941 when Germany and its allies occupied and partitioned the Kingdom of Yugoslavia, entire Syrmia region (including the left bank of the Sava) became part of the Independent State of Croatia. Nazi secret police, Gestapo, took over Sajmište. They encircled it with several rings of barbed wire turning it into what they referred to as "collection center" – a euphemism for a prison. It eventually became a concentration camp. Until May 1942 Germans used Sajmište concentration camp to mostly kill off Jews from Belgrade and other parts of Serbia. From April 1942 onwards, Serbian prisoners were transported in from Jasenovac and Stara Gradiška concentration camps run by Croatian Ustaše collaborators. Partisans captured throughout Serbia were also sent to Sajmište. Detainees were also sent in from other parts of Yugoslavia, especially Serbs after major German offensives on briefly liberated territories. Executions of captured prisoners lasted as long as the camp existed. During their heavy “Easter bombing” of Belgrade, Allied aircraft bombed Sajmište on 17 April 1944, killing some 100 inmates and inflicting heavy damage on the camp itself, destroying all the buildings except for the Spasić pavilion and the Central tower..