Short answers, pulled from the story.
The NKVD prisoner massacres were a series of mass executions of political prisoners carried out by the NKVD, the Soviet Union's People's Commissariat for Internal Affairs, across Eastern Europe, primarily in Poland, Ukraine, the Baltic states and Bessarabia. They took place after the German invasion of the Soviet Union began, when the NKVD was ordered to evacuate prisoners but often executed them instead.
The massacres began after the German invasion of the Soviet Union on the 22nd of June 1941, and continued over the following weeks, with killings recorded as late as October and November 1941 in places such as Simferopol and Yalta.
Estimates put the total at approximately 100,000 extrajudicial executions in the span of a few weeks, including nearly 9,000 deaths in the Ukrainian SSR and between 20,000 and 30,000 in what was then eastern Poland.
The killings occurred across Eastern Europe, primarily in Poland, Ukraine, the Baltic states and Bessarabia, with mass shootings documented in Belarus, Estonia, Lithuania, Poland, Ukraine and Russia. Postwar investigators in Germany, Poland, Belarus and Israel later identified at least 25 prisons where prisoners were killed.
NKVD troops were ordered to evacuate political prisoners into the Soviet interior, but the hasty retreat of the Red Army, a lack of transportation and other supplies, and general disregard for legal procedure often led to prisoners being executed instead.
In Lviv, the massacres began on the 22nd of June 1941, the day of the German invasion, and continued until June 28, with the NKVD executing several thousand inmates across the city's provisional prisons. An estimated 4,000 people were murdered, and only around 270 survivors are known.