The Nigerian-born Islamic militant group Boko Haram is terrorising communities inside neighbouring Cameroon and snatching young boys from across the border and forcing them to join the sect.
Abubakar Shekau is the shadowy leader of Boko Haram who took control of the Islamist group after the death of founder Mohammed Yusuf in 2009.
Little is known about him, although he was born in Shekau village in the northeastern state of Yobe and is now thought to be in his early 40s.
We saw abandoned villages and burned-out schools inside Cameroon, despite the presence of hundreds of troops including some of the country's top soldiers from the elite rapid response unit Battalion D'Intervention Rapide (BIR).
Shekau is Nigeria's most-wanted man and was designated a terrorist by the US government in 2012.
The huge 1,243-mile (2,000km) border with Nigeria is mostly unmanned and un-policed, allowing Boko Haram to cross over and mount attacks inside Cameroon with horrifying regularity.
A reward of $7m (£4.6m) and 50m Nigerian naira (£182,000) has been issued for information leading to his location.
Shekau is also known as "Darul Tawheed", a reference to his knowledge of an orthodox doctrine of Islam centred on the oneness of Allah.
Soldiers from the BIR are desperately trying to stop the spread of Boko Haram in their country.
Nigerian authorities thought he had been killed in 2009 during clashes with security forces but he reappeared in a video in 2010 to claim leadership of Boko Haram.