Round-ups and killings of Jews began immediately following the arrival of the first German troops in 1941, who were closely followed by the extermination squad Sonderkommando 1a under Martin Sandberger, part of Einsatzgruppe A led by Walter Stahlecker. Arrests and executions continued as the Germans, with the assistance of local collaborators, advanced through Estonia. Unlike German forces, Estonians seem to have supported the anti-Jewish actions on the political level, but not on a racial basis. The standard excuse used for the "cleansing" operations was arrest 'because of Communist activity'. This equation of Jews with Communism evoked a positive Estonian response, and attempts were made by Estonian police to determine whether an arrested person indeed supported Communism. Estonians often argued that their Jewish colleagues and friends were not communists and submitted proof of pro-Estonian conduct in the hope of being able to get them released. Anton Weiss-Wendt in his dissertation "Murder Without Hatred: Estonians, the Holocaust, and the Problem of Collaboration" concluded on the basis of the reports of informers to the occupation authorities that Estonians in general did not believe in Nazi anti-Semitic propaganda and by majority maintained positive opinion about Jews. Estonia was declared "Judenfrei" quite early, at the Wannsee Conference on 20 January 1942, as the Jewish population of Estonia was small (about 4,500), and the majority of it managed to escape to the Soviet Union before the Germans arrived. Virtually all the remainder (921 according to Martin Sandberger, 929 according to Evgenia Goorin-Loov and 963 according to Walter Stahlecker) were killed. The Nazi regime also established 22 concentration and labor camps in Estonia for foreign Jews, the largest being Vaivara concentration camp. Several thousand foreign Jews were killed at the Kalevi-Liiva camp. An estimated 10,000 Jews were killed in Estonia after having been deported there from Eastern Europe.