The darkest page in the monastery's history occurred in the fall of 1939, when some 5,000 captured Polish officers were sequestered there in what was called concentration camp Kozelsk-1. Walled monasteries were frequently used as prisons during this period. In 1940 this group was sent to Katyn and shot in a mass execution. Following a brief period as a military hospital, the monastery was used in 1944-1945 as an NKVD "filtration camp" for Soviet officers repatriated from German camps. In later years the territory served as an agricultural school. The Presentation Optina Pustyn Monastery was finally returned to the Orthodox Church in 1987 and services resumed in 1988. In 1990 the John the Baptist Skete was also returned to the monastery. Thus began a process of restoration whose results are so impressively visible today. Like the Solovetsky Monastery in the Russian North, which also witnessed tragic events during the Soviet era, Optina Pustyn has experienced a revival that each year attracts thousands of pilgrims and visitors.