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Who are Boko Haram?(http://news.bbcimg.co.uk/media/images/78990000/jpg/_78990678_024662742.jpg) The Boko Haram leader has declared a caliphate in areas he controls
- Founded in 2002
- Initially focused on opposing Western education - Boko Haram means "Western education is forbidden (http://www.bbc.co.uk/news/blogs-magazine-monitor-27390954)" in the Hausa language
- Launched military operations in 2009 to create Islamic state
- Thousands killed, mostly in north-eastern Nigeria - also attacked police and UN headquarters in capital, Abuja
- Some three million people affected
- Declared terrorist group by US in 2013
Who are the militants? (http://www.bbc.co.uk/news/world-africa-13809501)
(http://news.bbcimg.co.uk/media/images/75306000/jpg/_75306515_line976.jpg) Since the kidnapping people have complained that the area was not well protected and many residents of Chibok had already moved to safer parts of the country, fearing another attack.
The BBC's Will Ross reports from Nigeria that the military has repeatedly failed to defend towns and villages in the north-east, allowing Boko Haram to steadily expand the area they control.
The crisis in Nigeria is deepening every week, but politicians appear more focused on next year's elections, our correspondent says.
Boko Haram has changed tactics in recent months by holding on to territory rather than using hit-and-run attacks that have left thousands dead.
Last month, the group dismissed the government's claims to have agreed a ceasefire. The government had said the ceasefire would set the stage for the release of the Chibok schoolgirls.
Chibok is a relatively small, mainly Christian town.
Its seizure comes a day after a government helicopter came down in Yola, in north-east Adamawa State - the second military helicopter to go down in the area in a week.
Also on Thursday, officials and residents said the army had managed to recapture the town of Mubi, which was the biggest town under Boko Haram's control.
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Source: BBC