Lviv (Polish: Lwów) was a multicultural city just before World War II, with a population of 312,231. The city's 157,490 ethnic Poles constituted just over 50 percent of the population, with Jews at 32 percent (99,595) and Ukrainians at 16 percent (49,747). On 28 September 1939, after the joint Soviet-German invasion, the USSR and Germany signed the German–Soviet Frontier Treaty, which assigned about 200,000 km2 (77,000 sq mi) of Polish territory inhabited by 13.5 million people of all nationalities to the Soviet Union. Lviv was then annexed to the Soviet Union. At the time of the German attack on the Soviet Union on 22 June 1941, about 160,000 Jews lived in the city; the number had swelled by tens of thousands due to the arrival of Jewish refugees from German-occupied Poland in late 1939. All along the German-Soviet front, the Soviet secret police (the NKVD) engaged in mass murder of prisoners, in what later became known as the NKVD prisoner massacres. According to estimates by contemporary historians, the number of victims in Western Ukraine was probably between 10,000 and 40,000, with at least two-thirds of them ethnic Ukrainians.