Jasenovac concentration camp
There are many different documents from the time that Jasenovac existed, written by many different people.  At the time, the Germans and Italians were fighting Yugoslav Partisans for control of Yugoslavia.  People from both sides of that fight wrote about Jasenovac.  The Ustaše themselves also wrote about the camp; so did the Vatican. Comparing all of these different accounts can help with estimates of how many people died at the camp.