During the invasion of Poland starting World War II, Nazi Germany carried out air raids on September 1–2, 1939, and then German troops entered the city on September 6. Afterwards, the German "Einsatzgruppe I" entered the city to commit various atrocities against the population, and then its members co-formed the local German police and security forces. Under German occupation the city was made part of the General Government. Poles expelled in December 1939 from several villages in the German-annexed Sieradz County were deported in freight trains to Nowy Sącz, while many locals were among Poles imprisoned in the infamous Montelupich Prison in Kraków and then murdered in the Krzesławice Fort of the Kraków Fortress, as part of the "Intelligenzaktion". Because of its proximity to Slovakia, it lay on a major route for resistance fighters of the Polish Home Army. The Gestapo was active in capturing those trying to cross the border, including the murder of several Polish pilots. In June 1940, the resistance rescued Jan Karski from a hospital there, and a year later 32 people were shot in reprisal for the escape; several others were sent to concentration camps.