Jasenovac concentration camp
How long the camp existed: Jasenovac existed from mid-August 1941 to May 1945.  Mass extermination took place during all of 1941 and 1942, and again in the second half of 1944.  From March to December 1943, almost no mass atrocities took place.  However, deaths from illness and individual killings continued (any guard could still kill any inmate at any time).