Here is a snapshot of key events since the abduction of more than 200 schoolgirls by Boko Haram jihadists from the remote town of Chibok in northeastern Nigeria more than two years ago.
Nigeria confirmed Thursday that 21 girls had been released following a prisoner exchange deal struck with Boko Haram.
– Snatched from school –
On April 14, 2014, gunmen from the Islamist group Boko Haram seize 276 girls from the Government Girls Secondary School in Chibok, Borno state.
The girls are forced from their dormitories onto trucks and driven into the bush. Fifty-seven girls manage to flee.
An international media campaign is launched, backed by personalities including US First Lady Michelle Obama and Nobel Peace Prize laureate Malala Yousafzai.
The hashtag #BringBackOurGirls fires a social media storm that ultimately achieves little.
Boko Haram leader Abubakar Shekau claims responsibility for the mass abduction in a video released on May 5, and vows to sell the girls as slave brides.
One week later, a second video shows about 100 of the missing girls. Boko Haram says they have converted to Islam and will not be released unless militant fighters held in custody are freed.
– Global response –
On May 17, Benin, Cameroon, Chad, Niger and Nigeria vow to fight Boko Haram together, in what Cameroon President Paul Biya terms a “declaration of war”.
The UN Security Council says the kidnappings “may amount to crimes against humanity,” as Britain, China, France, Israel and the US offer help.
US military specialists deploy to neighbouring Chad but later move elsewhere after Nigeria stops requesting their services.
On May 26, Nigeria’s Chief of Defence Staff Alex Badeh says the girls have been located but warns a rescue operation would put their lives at risk.
– ‘Married off’ –
On October 31, Shekau quashes rumours of a deal with Nigerian authorities and says the girls have converted to Islam and been “married off”.
– One year on –
On April 14, 2015, Nigeria’s president-elect Muhammadu Buhari warns he “cannot promise that we can find” the girls, as vigils are held in many countries to mark their first year in captivity.
Amnesty International believes the girls have been separated into three or four groups and are being held in camps, some of which might be in Cameroon or Chad.
Buhari says in late December he is willing to negotiate with any “credible” Boko Haram leadership, a week after claiming the country has “technically” won the war against Boko Haram.
– Others freed –
Throughout 2015, the Nigerian military announces the rescue of hundreds of people, most of them women and children, who have been kidnapped by Boko Haram.
But the missing schoolgirls are not among them, despite several unconfirmed sightings.
Suicide attacks using women and young girls increase against “soft” civilian targets such as mosques, markets and bus stations, fuelling fears that Boko Haram might be using its captives.
In March 2016, it emerges that Boko Haram also seized 500 women and children from the north east town of Damasak in Borno state just months after the Chibokabduction. The kidnapping was denied at the time.
– ‘Proof of life’ –
On April 13, 2016, US news channel CNN reports that Boko Haram has sent a “proof of life” video which shows 15 of the girls, the first concrete indication that at least some are still alive.
On May 18, 2016 the Nigerian army confirms the first of the schoolgirls has been found. The 19-year-old, who later met President Buhari, was discovered with a four-month-old baby and a man she described as her husband near Boko Haram’s Sambisa Forest enclave.
– Prisoner exchange –
On October 13, 2016, Nigerian officials announce the release of 21 of the girls following talks between the government and Boko Haram brokered by Switzerland and the International Red Cross. Local sources say four jihadist prisoners were released as part of the deal to secure the girls’ release.
The Nigerian government has raised the prospect that more releases could follow, with a senior official in the president’s office saying that “the negotiations will continue”.