Unlike in other Nazi-occupied countries where the Holocaust was introduced gradually (first limiting Jewish civil rights, then concentrating Jews in ghettos, and only then executing them in death camps), executions in Lithuania started on the first days of war. Einsatzkommando A entered Lithuania one day behind the Wehrmacht invasion to encourage self-cleansing. According to German documents, on June 25–26, 1941, "about 1,500 Jews were eliminated by Lithuanian partisans. Many Jewish synagogues were set on fire; on the following nights another 2,300 were killed." The killings provided justification for rounding up Jews and putting them in ghettos to "protect them", where by December 1941 in Kaunas, 15,000 remained, 22,000 having been executed. The executions were carried out at three main groups: in Kaunas (Ninth Fort), in Vilnius (Ponary massacre), and in countryside (Rollkommando Hamann). In Lithuania, by 1 December 1941, over 120,000 Lithuanian Jews had been killed. It is estimated that 80% of the Lithuanian Jews were killed before 1942, many by or with the active participation of Lithuanians in units, such as Police Battalions.