Jasenovac concentration camp
The Nazis had used poison gas to kill many prisoners in their concentration camps.  Following this example, the Ustaše tried to use poison gas to kill inmates that arrived in Stara-Gradiska.  At first, they tried to gas the women and children that arrived from camp Djakovo with gas-vans that Simo Klaić called "green Thomas".  Later, they built gas chambers and used Zyklon-B and sulphur monoxide to kill prisoners.