The Jewish population of Sanok may have been almost 30% for many years in the early 20th century. During the joint German–Soviet invasion of Poland, which started World War II, in September 1939, it was invaded by Germany, and the "Einsatzgruppe I" entered the town on September 25, 1939 to commit various atrocities against the populace. In 1939–1940, the Germans imprisoned many Poles in the local prison, especially those who tried to escape occupation to take refuge in Hungary. The Germans then massacred 112 Poles at the Gruszka mountain near Tarnawa Dolna. The victims are buried at the Central Cemetery in Sanok. At the beginning of the German occupation during World War II, the Jewish population was around 5,000. During the occupation, most of the Jews were either executed or killed in Nazi death camps or Nazi concentration camps during the Holocaust. Some of the actions against the Jews were assisted by Ukrainian auxiliaries and hundreds of the deaths occurred in Sanok itself, while the Polish resistance movement established the secret Polish Council to Aid Jews "Żegota", which operated in the town. Buildings that had been owned by Jews were taken by the local population. The local Jewish cemetery still exists. Several hundred Jews are thought to have survived, most of whom fled to the Soviet Union at the beginning of the war. Some of the Jews emigrated to Canada and the United States in the early 1900s with Sanoker Burial Societies spreading throughout New York and other regions where they settled.