Was Nigeria's military warned that hundreds of schoolgirls were about to be kidnapped?

Amnesty International is blasting Nigeria's top military leaders, claiming they knew four hours in advance that Boka Haram militants were going to raid the Chibok boarding school. The attack ended with nearly 300 girls kidnapped at gunpoint. There has been no sign of them since, and Boko Haram's leader has said he will sell the girls into slavery. Vladimir Duthiers reports on why finding them gets more difficult by the day.