In 1939, after the invasion of Poland, which started World War II, Gdańsk Pomerania was annexed by Nazi Germany and incorporated into the newly formed province of Reichsgau Danzig-West Prussia. During the German occupation, the Polish population was subjected to mass arrests, repressions, deportations to Nazi concentration camps, expulsions and massacres. Mass arrests especially pertained to Polish teachers and clergy, and were carried out in September and October 1939. 16 priests were murdered at the local shooting range, and others were imprisoned in Tczew, and later murdered during large massacres of Poles in the Szpęgawski Forest. On October 20, 1939, 44 inhabitants were murdered in the Szpęgawski Forest. The Germans removed traces of Polish heritage from the cathedral and the seminary, and also looted or destroyed historic Polish collections, including the library and the archive. The "Einsatzkommando 16" established a prison for local Polish activists, teachers and priests at the seminary, and the victims were later either murdered in the Szpęgawski Forest, deported to concentration camps, or moved to the prison in Starogard Gdański. In 1940 the Germans shortly operated a subcamp of the Stutthof concentration camp in the town.