Auschwitz-Birkenau became the killing centre where the largest numbers of European Jews were killed. By mid 1942, mass gassing of Jews using Zyklon-B began at Auschwitz, where extermination was conducted on an industrial scale with some estimates running as high as three million persons eventually killed through gassing, starvation, disease, shooting, and burning. 9 out of 10 were Jews. In addition, Gypsies, Soviet POWs, and prisoners of all nationalities died in the gas chambers.