Łomża and Siedlce, to commit various atrocities against Poles during the German invasion of Poland that began World War II in 1939. German troops invaded Poland also from Olsztyn (then called Allenstein). After the German invasion of Poland, local Poles were also subjected to mass executions and deportations to occupied Poland. Arrested Poles were held in a local prison and then forced to remove Polish signs and inscriptions in the city, while the German population gathered and insulted them. The Gazeta Olsztyńska was abolished by the German authorities, the newspaper's headquarters was demolished and the editor-in-chief Seweryn Pieniężny was arrested and executed in the along with co-publisher Wojciech Gałęziewski and the "Rolnik" Cooperative chief Leon Włodarczyk, while Pieniężny's wife was deported to the Ravensbrück concentration camp. The last pre-war Polish consul in Allenstein, Bohdan Jałowiecki, along with the consulate staff, was imprisoned in the Hohenbruch and Soldau concentration camps, and then murdered. Polish teachers were deported to the Dachau concentration camp. During the war five forced labour camps were established in the city. On 12 October 1939, the Wehrmacht established an area headquarters for one of its military districts, Wehrkreis I (headquartered at Königsberg), that controlled the environs of Allenstein, including Lötzen (now Giżycko), and Ciechanów in occupied Poland. As part of the Aktion T4, Nazi Germany conducted medical experiments on the patients of the psychiatric hospital in the present-day district of Kortowo, in which at least 5,000 people were killed. On 22 January 1945, near the end of the war, the city was plundered and burned by the conquering Soviet Red Army, and much of its German population fled. The remaining population was subjected to various crimes, including murder, rape and looting. The Soviets also murdered the remaining patients and staff of the psychiatric hospital, who were either burned alive or shot. Remains of three Roman Catholic nuns who served as nurses at Olsztyn's St. Mary's Hospital and were killed by Soviet soldiers in 1945 were excavated in October 2020. On 23 May 1945, the Soviets established a Polish administration in the region which aroused British and American protest. The Polish rule was accepted under the preliminary provisions of the Potsdam Conference. In October 1945, the remaining German population was expelled, to be replaced by new Polish settlers, mostly those expelled from pre-war Polish regions of Vilnius, Grodno and Volhynia, annexed by the Soviet Union, as well as settlers