Chibok is the town in northeast Borno state from which more than 200 girls were abducted in April. The insurgents abducted many soldiers who remain missing, they said. Boko Haram extremists are demanding the release of detained fighters in return for the kidnapped girls. Image caption Boko Haram has said it will not free the girls until authorities release all imprisoned militants 
 
 Nigeria's military says it has raided a Boko Haram intelligence unit thought to be linked to the recent abduction of more than 200 schoolgirls. The militants have increased the tempo and deadliness of attacks this year, with more than 2,000 people estimated killed compared to 3,600 in all four previous years. Image copyright AFP Image caption Boko Haram leader Abubakar Shekau has been designated a terrorist by the US government 
 
 Founded in 2002 
 
 Initially focused on opposing Western education - Boko Haram means "Western education is forbidden" 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 Boko Haram? The militants have been blamed for several other recent attacks, including an assault on villages near Chibok on Sunday that killed at least 40 people, and a bomb attack on a busy shopping district in Nigeria's capital, Abuja, last week.
– Nigeria's military says it has arrested a businessman who belonged to a vigilante group fighting Boko Haram but was secretly a member of the extremist group himself—and "participated actively" in the kidnap of more than 200 schoolgirls in April. The military says the businessman was part of a Boko Haram intelligence unit and coordinated attacks that have killed hundreds of people, the BBC reports. Two women from the intelligence unit have also been arrested. It's not clear whether the arrest of the businessman will help rescue the girls, at least 219 of whom are still believed to be in captivity: Nigeria said a month ago that it knows where the girls are, but fears trying to free them with force could get them killed. Boko Haram, which says it won't free the schoolgirls until captured fighters are freed, has killed more than 2,000 people so far this year, including dozens killed Sunday in attacks on churches just a few miles from where the girls were kidnapped, the AP reports. Vigilante groups say the militants gunned down worshippers before torching at least four churches, killing more than 30 people.