Nigeria’s military had advance warning of the attack on a school at which some 270 girls were kidnapped but failed to act, a rights group says.
Several credible sources have told Amnesty International that the military in the state capital, Maiduguri, had more than fours hours’ warning.
Fifty-three of the girls have escaped, leaving more than 200 still captive.
Teams of experts from the US and UK have arrived in Nigeria to help locate and rescue the abductees.