( CNN ) -- It 's been more than three weeks since militants from the dreaded Boko Haram terrorist group dragged 276 girls out of their beds at a boarding school in northern Nigeria , and still no one knows where the girls are . International assistance has begun to flow into Nigeria , whose president has vowed to end the terror threat plaguing his country . Here 's what you need to know to get caught up : . Where are the girls ? It 's anyone 's guess . Boko Haram 's leader , Abubakar Shekau , said in a video that he was going to sell them into slavery , but it 's unknown whether he has . Pentagon spokesman U.S. Navy Rear Adm. John Kirby and other officials have said they believe the girls may have been separated into smaller groups , making the task of finding them inordinately more difficult . Gordon Brown , a former UK prime minister and the U.N. 's special envoy for global education , speculated that the girls may have been moved into neighboring countries . `` The search must be in Niger , Cameroon and Chad , to see if we can find information , '' he said . What 's being done to find them ? Nigeria has n't given a lot of information about its efforts other than to say that its soldiers have been out in the field , looking for the girls . Nigerian police offered a $ 310,000 reward , but there 's no evidence that has turned up any leads . The United States and Britain have sent advisers to help the Nigerian government find the girls , stage rescue missions and help in the larger fight to defeat Boko Haram . Has Nigeria done enough ? Not by a long shot , according to many critics . Amnesty International came out Friday with a damning report accusing Nigerian officials of having warnings about the raid on the school in Chibok but failing to react . Nigerian President Goodluck Jonathan has been roundly criticized for waiting three weeks to speak to the nation on efforts to find and free the girls . And larger efforts to subdue Boko Haram have had little success since Jonathan declared a state of emergency in Nigeria 's north one year ago this month . What is Boko Haram ? Nigerian and U.S. officials call it a terrorist group dedicated to founding an Islamic state in Nigeria , which is divided between a largely Muslim north and a predominantly Christian south . It has roots , however , in deep-seated animosity in northern Nigeria over poor governance and anger over economic inequality . Most of Nigeria 's oil wealth is concentrated in the south . The group has been blamed for dozens of brutal attacks in Nigeria , including attacks on churches , schools and other targets . Last year , the group was blamed for an attack that killed more than 40 students at an agricultural college . In February , the group burned down a school in Yobe state , killing 29 boys . Last month , in addition to the school abductions , the group claimed responsibility for the bombing of an Abuja bus station that left dozens of people dead . And on Monday , Boko Haram fighters attacked a town that had been used as a staging ground for troops looking for the girls . At least 310 people were killed , some of them burned alive . Why did they do this ? It 's all in the group 's name . Boko Haram translates to `` Western education is sin '' or `` forbidden . '' The group has called for an end to all such education , especially for girls . In a video that surfaced this week , Shekau said girls should n't be in school . `` Women must go and marry , '' he said . What 's going to happen to the girls if government forces do n't find them ? Shekau said he was going to sell them into slavery . `` Allah says I should sell . He commands me to sell , '' he said in the video . Some experts are n't so sure they will be sold outright , but they could used as sex slaves for fighters , forced into marriages or traded for ransom . They may also be used as human shields in any military operation against Boko Haram , said Shehu Sani , a human rights activist in northern Nigeria . Video purports to show abducted Nigerian girls . The road to Boko Haram 's heartland . CNN 's Aminu Abubakar , Josh Levs , Vladimir Duthiers , Faith Karimi , Elise Labott , Chelsea Carter , Paul Cruickshank and Tim Lister contributed to this report .
Nigeria has struggled against Boko Haram for years . The group is believed to have killed more than 20,000 people in the past two years. Military advisers and other experts are now in Nigeria to help plan terror fight .