Adjustments to the secret protocols of the 1939 Molotov-Ribbentrop Pact assigned Lithuania to the Soviet sphere of influence, and it was occupied by the USSR in June 1940. The fortress was then used to conduct interrogations and house political prisoners. The pact was broken when Germany invaded USSR on June 22, 1941. German forces entered Kaunas on June 24. The Sixth Fort became a POW camp for Red Army soldiers. Kaunas's Jewish population numbered between 35,000 and 40,000; few would survive the Holocaust in Lithuania. The Nazis, aided by Lithuanian auxiliaries, began massacring the Jewish population. On July 6, acting under orders of the SS, Lithuanian auxiliary police units shot nearly 3,000 Jews at the Seventh Fort. On August 18, in what came to be known as the "intellectuals action", over 1,800 Jews were shot at the Fourth Fort. On October 28, the "Great Action" took place—the residents of the Kaunas Ghetto were summoned, and over 9,000 men, women and children were taken to the Ninth Fort and executed. During the later course of the occupation, over 5,000 Jewish deportees from Central Europe would be executed at this fort. About 60 escaped in December 1943; they had been assigned to excavate and burn the bodies of earlier victims, as part of Aktion 1005. Thirteen of these escapees were able to document the Aktion's attempt to hide the evidence of the mass murders.