(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 hasn'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 shouldn't be in school. "Women must go and marry," he said.
What's going to happen to the girls if government forces don'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 aren'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.
CNN's Aminu Abubakar, Josh Levs, Vladimir Duthiers, Faith Karimi, Elise Labott, Chelsea Carter, Paul Cruickshank and Tim Lister contributed to this report.