MAIDUGURI, Nigeria (AP) — The abduction of almost 300 Chibok schoolgirls by Boko Haram extremists more than two years ago shocked the world. On Thursday, Nigeria's government announced that 21 of the girls had been freed after its negotiations with Boko Haram. It is the first release of any of the students as a result of government action. Here are some of the key events surrounding the crisis:

April 2014: Boko Haram extremists kidnap 276 schoolgirls from Chibok in northern Nigeria, the region where the insurgency emerged several years ago.

November 2014: Extremists seize Chibok, then Nigerian army takes back the town.

May 2015: New President Muhammadu Buhari is sworn in, pledges to tackle Boko Haram "head-on."

April 13, 2016: Boko Haram video appears to show some of the Chibok girls and tearful mothers recognize their daughters.

May 18, 2016: Relative says one of the Chibok girls is found, pregnant, in a forest; pressure grows on Nigeria's government to rescue the others.

Aug. 14, 2016: Boko Haram video says some Chibok girls killed in airstrikes, demands release of extremists in exchange for other girls' freedom.

Oct. 13, 2016: Spokesman for Nigeria's president confirms 21 Chibok girls have been freed, a result of government negotiations with Boko Haram.