Here's a great story about the Mau Mau resistance in Kenya in the 1950's from Radiolab.
Kenya was an English colony. In the 20th century, the Kikuyu, the major ethnic group in Kenya, swore a secret oath to throw out the English. They began killing cattle and over time began killing white families.
The murder of one English family and their young son, who they hacked to death, brought a response by the British that was unusually brutal.
The British forced many of the Kikuya men into concentration camps and tortured them in heinous ways, like applying hot coals to their eyeballs.
Researchers talked to many elderly Kikuyu about their incarceration by the British but these oral histories did not satisfy many historians who wanted to see documents that outline the incarceration.
Radiolab explains the amazing story about how these documents eventually came to light.