Short answers, pulled from the story.
The NKVD prisoner massacres began immediately after the German invasion of the Soviet Union on the 22nd of June 1941. NKVD troops received order No. 00803 to liquidate or evacuate over 140,000 prisoners in occupied eastern Poland during this period.
Estimates indicate that total extrajudicial executions reached approximately 100,000 within weeks of the invasion. Nearly 9,000 people died in the Ukrainian SSR alone while Eastern Poland saw between 20,000 and 30,000 deaths during the same period.
Over 4,000 people were murdered in Lviv between June 22 and June 28. In Kharkiv, 1,200 prisoners were burned alive in a single incident and at Lutsk Soviet tanks machine-gunned prisoners who lined up for promised amnesty.
NKVD units executed large numbers of prisoners using shooting grenades bayonets and starvation to kill inmates inside cells. Some victims were scalded alive or had their ears noses and fingers cut off before death while others died en masse due to hunger thirst and direct violence from their captors.
After World War II Germany Poland Belarus and Israel conducted official inquiries into the events. The Joffe Foundation maintains a directory listing 411 sites within Russia from the Civil War to the 1950s while researchers analyze data from sources such as Timofeev V.G.'s work on criminal-execution systems published in Cheboksary in 1999.