One day during the Nazi occupation of western Ukraine in the second world war, a young Jewish woman slipped out of the ghetto in the town of Rava Ruska to buy some butter in the market.

On her way, she was spotted by a German officer, who ordered her stripped naked, made the seller smear her body in butter, and then had her beaten to death with sticks.

This story was one of thousands relayed to a team of researchers led by Patrick Desbois, a French Catholic priest who has spent years investigating one of the most under-researched parts of the Holocaust.

Killings in western Ukraine were not carried out using the industrialised methods of Auschwitz and other death camps. Instead Jews were rounded up and shot, one by one. Sometimes they were kicked or beaten to death. No records were kept, so keeping track of numbers and locations is difficult.

Now, for the first time, monuments have been erected in Rava Ruska and four other towns in west Ukraine to properly commemorate the killings. In four sites around Rava Ruska, around 15,000 Jews were killed in total, Desbois said at the opening ceremony. Until four years ago, villagers would regularly turn up bones when planting vegetables or simply walking in the woods outside the town. ...