In September 1939, after the outbreak of World War II, German Luftwaffe planes bombed Zamość several times. Over 250 people were killed, mainly civilians. The city was overrun by the Germans during the invasion of Poland and the local garrison, staffed by the Polish infantry regiment of "podpułkownik" Stanisław Gumowski, was defeated. On September 27, 1939, Nazi Germany signed a border treaty with the Soviet Union which had invaded Poland from the east, and, consequently, on September 28, 1939, Zamość was handed over to the Red Army – for about a week. The Soviets withdrew on October 5, 1939, along with some 5,000 Jews after a further demarcation line adjustment. The Germans returned to the city on October 8, 1939 and shortly afterwards mass arrests of prominent citizens began. This was as part of the secret A-B Action, the deliberate extermination of Polish intellectuals. The German Nazis created an execution site in the Zamość Rotunda, Gestapo camp, (in German: "Gefangenen-Durchgangslager Sicherheitspol", in English: "The transit camp for Security Police prisoners"). More than 8,000 people were massacred there, including displaced residents of the region. In Zamość, Nazi Germans also created a "Transit Camp" on Okrzei Street, for arrested and displaced inhabitants of the Zamość region (including thousands of children) and camps of Soviet prisoners of war captured during Operation Barbarossa.