The Nigerian military knew that a "convoy" of armed Boko Haram militants was near the town of Chibok—where more than 200 girls were eventually kidnapped—for at least four hours before the attack actually occurred, Amnesy International says. From their press release:
Amnesty International has confirmed through various sources that Nigeria's military headquarters in Maiduguri was aware of the impending attack soon after 7:00 p.m. on April 14, close to four hours before Boko Haram began their assault on the town.
But an inability to muster troops - due to poor resources and a reported fear of engaging with the often better-equipped armed groups - meant that reinforcements were not deployed to Chibok that night. The small contingent of security forces based in the town - 17 army personnel as well as local police - attempted to repel the Boko Haram assault but were overpowered and forced to retreat. One soldier reportedly died.
According to Amnesty, warnings were received from multiple civilians and local officials who had seen or learned about the militants headed toward Chibok. But, apparently, no army reinforcements ever reached the town despite the fact that the kidnappers did not leave until dawn of the next day.