A human rights group says Nigerian security forces had advance warning about the raid on a girls boarding school in which militants kidnapped more than 300 students.
Amnesty International says Nigerian security forces failed to act on warnings that began coming in more than four hours before Boko Haram fighters attacked the school in the town of Chibok late on April 14.
The rights group blamed the lack of action on an "inability to muster troops," due to poor resources and a possible fear of engaging with the often better-armed militants.
Amnesty said Friday that it learned of the warnings from local officials and two senior military officers. There was no immediate reaction from the Nigerian government.
At least 11 U.S. military staff officers have arrived in Nigeria to help find the 276 girls still missing.
On Thursday, U.S. Secretary of State John Kerry said the United States will do all it can to free the girls.
"Our inter-agency team is hitting the ground in Nigeria now, and they are going to be working in concert with [Nigerian] President Goodluck Jonathan's government to do everything that we possibly can to return these girls to their families and their communities. We are also going to do everything possible to counter the menace of Boko Haram."
Britain, China, France and Canada have also promised to help with the search.
Boko Haram's leader has said the group intends to "sell" the girls. Nigerian police have offered a $300,000 reward for information leading to the location and rescue of the students.
Boko Haram is blamed for thousands of deaths since it launched an uprising against the Nigerian government in 2009.
The group has attacked schools, churches, mosques, police stations, and markets while seeking to establish strict Islamic law in Muslim-majority northern Nigeria.
Nigeria declared a state of emergency in three northeastern states a year ago but efforts by the military to crush or contain the insurgency have been unsuccessful.