During World War II, famous director Alfred Hitchcock worked with British media mogul Sidney Bernstein to have camera operators record the atrocities of the Holocaust, Addicting Info reported.

The images collected were so gruesome and disturbing that the footage ended up locked in the archives of the Imperial War Museum.

The footage was eventually restored and a shortened version that was missing several reels of footage aired on PBS in 1985.

A documentary titled Night Will Fall looks to answer the question of why the original documentary was so hidden. The film will air on HBO later this month and will be available on DVD in February.

Watch the documentary’s trailer.