https://www.bbc.com/news/world-europe-42144186
http://www.auschwitz.org/en/history/auschwitz-and-shoah/the-extermination-procedure-in-the-gas-chambers/
https://encyclopedia.ushmm.org/content/en/article/gassing-operations
https://encyclopedia.ushmm.org/content/en/article/euthanasia-program
https://www.yadvashem.org/odot_pdf/Microsoft%20Word%20-%206236.pdf
https://www.thoughtco.com/zyklon-b-gas-chamber-poison-1779688