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One of the mothers of the missing Chibok school girls wipes her tears as she cries during a rally by civil society groups pressing for the release of the girls in Abuja on May 6, 2014, ahead of World Economic Forum. Members of civil society groups marched through the streets of Abuja and to the Nigerian defence headquarters to meet with military chiefs, to press for the release of more than 200 Chibok school girls abducted three weeks ago. Suspected Boko Haram Islamists have kidnapped eight more girls from Nigeria's embattled northeast, residents said on May 6, after the extremist group's leader claimed responsibility for abducting more than 200 schoolgirls last month and said in a video he was holding them as "slaves" and threatened to "sell them in the market". (PIUS UTOMI EKPEI/AFP/Getty Images) ( 529533573 )

Brig. Gen. Chris Olukolade, Nigeria's top military spokesman, centre, walks with representatives of kidnapped schoolgirls of Chibok secondary school, for a meeting at the defense headquarters, in Abuja, Nigeria, Tuesday, May 6, 2014. Their plight — and the failure of the Nigerian military to find them — has drawn international attention to an escalating Islamic extremist insurrection that has killed more than 1,500 so far this year. Boko Haram, the name means "Western education is sinful," has claimed responsibility for the mass kidnapping and threatened to sell the girls. The claim was made in a video seen Monday. The British and U.S. governments have expressed concern over the fate of the missing students, and protests have erupted in major Nigerian cities and in New York. (AP Photo/Sunday Alamba) ( Anatomy of a Kidnapping )

Members of civil society groups sit to protest the abduction of Chibok school girls during a rally pressing for the girls' release in Abuja on May 6, 2014, ahead of World Economic Forum. Members of civil society groups marched through the streets of Abuja and to the Nigerian defence headquarters to meet with military chiefs, to press for the release of more than 200 Chibok school girls abducted three weeks ago. Suspected Boko Haram Islamists have kidnapped eight more girls from Nigeria's embattled northeast, residents said on May 6, after the extremist group's leader claimed responsibility for abducting more than 200 schoolgirls last month and said in a video he was holding them as "slaves" and threatened to "sell them in the market". (PIUS UTOMI EKPEI/AFP/Getty Images) ( 529533765 )

A soldier takes position to stop the advancing civil society groups protesting the abduction of Chibok school girls during a rally pressing for the girls' release in Abuja on May 6, 2014, ahead of World Economic Forum. Members of civil society groups marched through the streets of Abuja and to the Nigerian defence headquarters to meet with military chiefs, to press for the release of more than 200 Chibok school girls abducted three weeks ago. Suspected Boko Haram Islamists have kidnapped eight more girls from Nigeria's embattled northeast, residents said on May 6, after the extremist group's leader claimed responsibility for abducting more than 200 schoolgirls last month and said in a video he was holding them as "slaves" and threatened to "sell them in the market". (PIUS UTOMI EKPEI/AFP/Getty Images) ( 529534707 )

In this Monday April 21, 2014, file photo, four female students of government secondary school Chibok, who were abducted by gunmen and reunited with their families, walk together in Chibok, Nigeria. They are among the 53 girls who escaped abduction by Islamic extremist kidnappers. The girls did not want to be identified by name and refused to speak to reporters when they were photographed in Chibok, the town from which the mass kidnapping of more than 300 girls took place on April 15. (AP Photo/Haruna Umar, File) ( Anatomy of a Kidnapping )

Tiffani Brown, left, joins a march in front of the Nigerian embassy in northwest Washington, Tuesday, May 6, 2014, protesting the kidnapping of nearly 300 teenage schoolgirls, abducted from a school in the remote northeast of Nigeria three weeks ago. (AP Photo/Manuel Balce Ceneta) ( Nigeria Kidnapped Girls )

A protester holds a sign during a demonstration May 6, 2014 outside the Nigerian embassy in Washington,DC demanding robust action to rescue more than 200 schoolgirls kidnapped by Boko Haram militants in the West African country. The State Department said the United States has offered to send a team of experts to Nigeria to help locate the young captives. (Robert MacPherson/AFP/Getty Images) ( 529534800 )