Kurt Gerstein wrote about what he saw in Belzec when he visited in 1942:

The chambers fill. "Pack well!" - Hauptmann Wirth has ordered. The people stand on each other's feet. 700 - 800 on 25 square metres, in 45 cubic metres! The SS physically squeezes them together, as far as is possible. The doors close. At the same time the others are waiting outside in the open air, naked. Someone tells me: "The same in winter!" "Yes, but they could catch their death of cold," I say. "Yes, exactly what they are here for!" says an SS man to me in his Low German. Now I finally understand why the whole installation is called the Hackenholt-Foundation. Hackenholt is the driver of the diesel engine, a little technician, also the builder of the facility. The people are brought to death with the diesel exhaust fumes. But the diesel doesn't work! Hauptmann Wirth comes. One can see that he feels embarrassed that that happens just today, when I am here. That's right, I see everything! And I wait. My stop watch has honestly registered everything. 50 minutes, 70 minutes [?] - the diesel doesn't start! The people are waiting in their gas chambers. In vain! One can hear them crying, sobbing... Hauptmann Wirth hits the Ukrainian who is helping Unterscharführer Hackenholt 12, 13 times in the face. After two hours and 49 minutes - the stop watch has registered everything well - the diesel starts. Until this moment the people live in these 4 chambers, four times 750 people in 4 times 45 cubic metres! Again 25 minutes pass. Right, many are dead now. One can see that through the small window in which the electric light illuminates the chambers for a moment. After 28 minutes only a few are still alive. Finally, after 32 minutes, everyone is dead!