Nigerian security officials failed to act on a warning about a militant group's raid on a boarding school that led to the abduction of more than 300 girls, Amnesty International says.
The human rights organisation cited "multiple interviews with credible sources," in asserting that Nigerian security forces had four hours of notice about the April 15 attack by Boko Haram in north-eastern Nigeria.
Amnesty says an inability to muster troops and fear of engaging with better equipped forces prevented forces from being deployed.
The Nigerian military's failure to find the girls has drawn international attention to an escalating Islamic extremist insurrection that has killed more than 1,500 people so far this year. Boko Haram, whose name means "Western education is sinful," has threatened to sell the girls.
Today British experts arrived in the Nigerian capital Abuja to help find the girls being held by Islamic militants in north-eastern Nigeria as an international effort began taking hold.
The experts were expected to work closely with US officials and agents in the search, the British government said as militants continued to stage attacks.
China and France have also promised help. Britain said its aim was not only to help with the current crisis but to defeat Boko Haram.
"The team will be considering not just the recent incidents but also longer-term counter-terrorism solutions to prevent such attacks in the future and defeat Boko Haram," the Foreign Office said in a statement.
A local government official confirmed that the Islamic extremists bombed a bridge linking the town of Gamboru to the Borno state capital, Maiduguri, the headquarters of the Nigerian military offensive. Gamboru was attacked on Monday by Boko Haram, leaving many dead. Estimates of the death toll from that attack ranged from 100 to as many as 300.
Communications with the remote town are difficult and it was not immediately possible to reconcile conflicting accounts of when the bridge was bombed.
The bombing of the bridge would prevent army convoys reaching Gamboru while leaving the way open for the insurgents to escape across a strategic bridge into neighbouring Cameroon - a bridge leading into mountains where the militants are known to have hideouts in caves.
The mass kidnapping of the schoolgirls has focused the world's attention on Boko Haram, and on the many civilian victims of the extremists.
President Goodluck Jonathan said at an economic forum on Thursday: "I believe that the kidnap of these girls will be the beginning of the end of terror in Nigeria."
However his government stands accused of being slow to mount operations to rescue the girls, who were kidnapped on April 15.
Boko Haram, which wants to impose Islamic law on Nigeria, abducted more than 300 girls from a boarding school in the north-east town of Chibok. On Thursday the government of Borno state, where Chibok is located, identified 53 girls who escaped, potentially subjecting the girls to stigma in this conservative society.
The government said in a statement today that the 53 girls it identified by name include those who fled the day they were kidnapped and those who escaped from Boko Haram camps days later.
Borno's government did not explain the decision to name the girls.
Chibok residents are staging a street protest today to press Borno's government to do more to find the missing girls.
Boko Haram has killed more than 1,500 people this year.