The Nigerian authorities were warned in advance that the militant group Boko Haram was en route to snatch more than 250 schoolgirls and failed to prevent the raid, according to Amnesty International.

The human rights organization said it had independently verified evidence that the Nigerian security forces had more than four hours’ warning that the attack was imminent and yet failed to mount an adequate defense of the boarding school in northern Nigeria.

Three weeks later, amid an international outcry for their release, the girls remain in captivity, and Islamist leaders have threatened to sell them as slaves in an African market.

Secretary of State John Kerry said U.S. personnel had arrived in Nigeria to help the rescue effort after the Nigerian government was criticized for its slow response.

It is now claimed that the Nigerian authorities could have intervened to spare the girls from the entire ordeal had they responded more robustly to advance warnings. Citing multiple sources in Nigeria, Amnesty claims the military headquarters in Maiduguri was aware of the planned attack by around 7 p.m. on April 14. The assault did not begin for another four hours.

“It amounts to a gross dereliction of Nigeria’s duty to protect civilians, who remain sitting ducks for such attacks,” said Netsanet Belay, Amnesty’s Africa director.

“The fact that Nigerian security forces knew about Boko Haram’s impending raid but failed to take the immediate action needed to stop it will only amplify the national and international outcry at this horrific crime,” he said. “The Nigerian leadership must now use all lawful means at their disposal to secure the girls’ safe release and ensure nothing like this can happen again.”

Whether through poor resources or a reluctance to engage directly with the heavily armed militant group, the authorities dispatched no reinforcements to Chibok that night, Amnesty alleges.

The small existing contingent of security forces based in the town, which reportedly consisted of 17 army personnel and local police, did try to repel the raid but were easily overpowered.

The damning claims will further increase the pressure on Goodluck Jonathan’s government in Abuja. Nigeria’s slow response—a “fact finding committee” was appointed less than a week ago—has already prompted protests around the world. The hashtag #BringBackOurGirls has been tweeted more than 1 million times, and U.S. first lady Michelle Obama is among those to have embraced the campaign to raise awareness of the plight of the children.

It is thought that as many as 50 of the schoolgirls escaped by jumping off the militants’ trucks as they were driven away, but 276 remained in captivity. At least two are believed to have succumbed to deadly snakebites.

Abubakar Shekau, the leader of Boko Haram, said he plans to sell the remaining girls as wives. “They are slaves and I will sell them because I have the market to sell them,” he said in a video claiming responsibility for the mass kidnapping. “God has commanded me to sell.”

American “experts” and British diplomatic and military officials have arrived to help scour the vast northern territories for the missing girls, who are aged between 12 and 15. The search is likely to be complicated if, as expected, the girls have been split into smaller groups to avoid detection.