Up to 4,000 people have died in Nigerian military custody since 2012, human rights investigators believe.
The Nigerian government has been fighting the Islamist group Boko Haram for the last two years.
Nigeria is now one of Africa’s largest economies. But the Muslim population in the north is among the poorest. That’s where Boko Haram has its stronghold.
The Nigerian government launched a “counter-terrorist” operation. It declared a state of emergency in three northern states.
Thousands of troops have since been sent into the area.
The army also took control of local Muslim militia whose job is to identify Boko Haram members in their own communities.
They were given powers to detain anyone who they wanted.
And new footage disclosed to Channel 4 directly implicates the Nigerian military in abuse of detainees.