The 14,500 Polish army officers, police, gendarmes, and civilians taken prisoner by the Red Army when it invaded eastern Poland in September 1939 were held in three special NKVD camps and executed at three different sites in spring 1940, of which the one in Katyn Forest is the most famous. Another 7,300 prisoners held in NKVD jails in Ukraine and Belarus were also shot at this time, although many others disappeared without trace. The murder of these Poles is among the most monstrous mass murders undertaken by any modern government.