Stalin's occupation of terror in eastern Poland in 1939 brought what Jan Gross calls "the institutionalization of resentment", whereby the Soviets used privileges and punishments to accommodate and encourage ethnic and religious differences between Jews and Poles. There was an upsurge in the anti-Semitic stereotype of Jews as Communist traitors; it erupted into mass murder when Nazi Germany invaded Soviet eastern Poland in the summer of 1941. A group of at least 40 Poles with an unconfirmed level of German backing killed hundreds of Jews at the racially aggravated Jedwabne pogrom. There was a rash of other massacres of Jews across the same formerly Soviet-occupied region of Łomża and Białystok around the same time, with varying degrees of German death squad incitement or involvement: at Bielsk Podlaski (the village of Pilki), Choroszcz, Czyżew, Goniądz, Grajewo, Jasionówka, Kleszczele, Knyszyn, Kolno, Kuźnica, Narewka, Piątnica, Radziłów, Rajgród, Sokoły, Stawiski, Suchowola, Szczuczyn, Trzcianne, Tykocin, Wasilków, Wąsosz, and Wizna.