Nigerian military commanders knew that Boko Haram militants were on their way to raid a boarding school at least four hours before 276 girls were abducted last month, human rights group Amnesty International reported Friday.
But commanders weren't able to raise enough troops to respond and so left a contingent of 17 soldiers and a handful of police officers based in the town of Chibok to fend off the militants, the group reported, citing senior Nigerian military officials and "multiple interviews with credible sources."
Borno state Sen. Ahmed Zannah said Friday that the military sent reinforcements, but not until the militants were already in Chibok.
CNN was not able to confirm Amnesty's claims with Nigerian officials, who have frequently been criticized for failing to prevent Boko Haram's deadly attacks, particularly in northern Nigeria.
At least 2,000 people have died in violence in northern Nigeria this year alone, Amnesty said. The most recent Boko Haram attack killed at least 310 people in a town that had been used as a staging group for troops searching for the missing girls.
"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," Amnesty International's Africa director for research and advocacy, Netsanet Belay, said in a statement on the group's website.
"It amounts to a gross dereliction of Nigeria's duty to protect civilians, who remain sitting ducks for such attacks. 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."
U.S. and British officials have arrived in Abuja to supplement a U.S. team already on the ground there, according to officials.
They will help Nigeria's government look for the missing girls, plan rescue missions and advise on ways to subdue Boko Haram. But U.S. officials, at least, say they are unlikely to commit troops to combat operations.
"Our interagency team is hitting the ground in Nigeria now, and they are going to be working ... with President Goodluck Jonathan's government to do everything that we possibly can to return these girls," Secretary of State John Kerry said Thursday.
'Many soldiers are afraid'
According to Amnesty, civilian officials in a nearby town and leaders of an armed vigilante group organized by the military informed nearby military posts that armed militants had passed through on their way to Chibok hours before the April 14 assault on the boarding school.
The human rights group said an official in the village of Gagilam told its investigators that residents had spoken of strangers who passed through on motorcycles, saying they were on their way to Chibok. The group cited another official as saying the men had asked herders for directions to the school.
The group also cited unnamed senior Nigerian commanders as saying they were aware of the attack even before the calls from the civilian leaders and vigilante groups. But they weren't able to muster enough troops to respond, Amnesty cited the commanders as saying.
"There's a lot of frustration, exhaustion and fatigue among officers and (troops) based in the hotspots," Amnesty quoted one of the unnamed commanders as saying. "Many soldiers are afraid to go to the battle fronts."
Where are the girls?
As many as 200 Boko Haram fighters carried out the Chibok school raid, Amnesty reported, herding the girls out of bed under the cover of darkness after a firefight with the handful of security forces in the town.
Boko Haram's leader, Abubakar Shekau, took credit for the mass kidnappings in a video that surfaced this week. He said he planned to sell the girls into slavery.
A few escapees shared harrowing tales of escaping into a nearby forest.
U.S. Navy Rear Adm. John Kirby, who serves as Pentagon press secretary, said officials believe the girls "have been broken up into smaller groups" but declined to detail how they came to the conclusion. His sentiment has been echoed by others.
"The search must be in Niger, Cameroon and Chad, to see if we can find information," said Gordon Brown, a former UK prime minister and the U.N.'s special envoy for global education.
International outrage has escalated over the nation's largely ineffective effort to subdue Boko Haram.
Amnesty is not the first to accuse Nigeria of failing to take enough action to stop the Chibok raid or other attacks, or to stage a forceful enough response in the aftermath.
Jonathan waited three weeks before speaking to the nation on the matter. He said that rescue efforts were under way at the time but that they could not be disclosed publicly.
"In a hostage situation, time is of the essence," Kirby said. "We lost some time."
On Thursday, Jonathan predicted that the abductions would be Boko Haram's undoing.
"By God's grace, we will conquer the terrorists. I believe the kidnap of these girls
will be the beginning of the end for terror in Nigeria," Jonathan said at the World Economic Forum meeting.
The international effort to buttress that fight ratcheted up Friday with the arrival of U.S. and British advisers.
Six U.S. military advisers arrived Friday, a U.S. official told CNN. They will join a team of U.S. and British officials already in Nigeria, helping find the girls, planning rescue efforts and devising strategies to help subdue Boko Haram.
A British team drawn from the country's Department for International Development, the Foreign and Commonwealth Office and the Ministry of Defence also arrived in Abuja on Friday, the British foreign office said.
About 60 U.S. officials have been on the ground since before the kidnappings as part of counterterrorism efforts with Nigeria, a senior U.S. administration official told CNN. They have been holding meetings, getting resources into the country and making assessments with local authorities.
Their tasks include establishing a coordination cell to provide intelligence, investigations and hostage negotiation expertise.
France also said it is sending a team but didn't provide specifics on what expertise it will bring.
British satellites and advanced tracking capabilities also will be used, and China has promised to provide any intelligence gathered by its satellite network, Nigeria said.
There are no plans to send U.S. combat troops, Kirby said.
U.S. Rep. Ed Royce, chairman of the House Foreign Affairs Committee, said the assistance is crucial.
"It's clear the Nigerian forces are struggling in this fight," he said Friday. "Direct U.S. security assistance intelligence and advisers in the field can make a big difference in rescuing these girls and combating this threat."
House Speaker John Boehner said President Barack Obama is doing the right thing.
"Clearly, there is danger whenever we send troops almost any place in the world," he said.
"But I do think the President is taking the right step here to work with our allies to try to do everything we can to get these girls back to their families in a safe way," Boehner said.
Kerry said the U.S. would "do everything possible to counter the menace of Boko Haram" and called for world action to help.
"The entire world should not only be condemning this outrage but should be doing everything possible to help Nigeria in the days ahead," he said.