A team of Russian historians and archaeologists have used a Nazi bomber pilot’s photograph to help them pinpoint the location of mass graves in Moscow containing the remains of thousands shot by Stalin’s secret police.

The existence of a mass grave in the Kommunarka district in southwestern Moscow first came to light in the dying days of the Soviet Union when the KGB opened up its archives.

It was one of three killing fields in the city used by Stalin’s NKVD secret police in the 1930s.