It has been more than two years since 276 girls were abducted from a secondary school in north-eastern Nigeria by militant Islamist group Boko Haram.
Their kidnapping sparked one of the biggest global social media campaigns, with tweeters using the hashtag #BringBackOurGirls.
But most of the girls are still missing. BBC News looks at what we know.
How were they kidnapped?
On 14 April 2014, Boko Haram militants attacked a government secondary boarding school in Chibok, Borno state, where girls from surrounding areas had gone to take exams.
Many schools in the region had shut down. Boko Haram was targeting them because of their opposition to Western education, which the militants believe corrupts the values of Muslims.
But Chibok had not been attacked before, so it was felt safe to use the school for the important final year exams. Many of the pupils were Christians.
The gunmen arrived in the town late at night in a blaze of gunfire and headed for the school where they raided the dormitories and loaded 276 girls on to lorries.
Some managed to escape within hours of their kidnapping, mostly by jumping off the lorries and running off into the bushes.
In total 219 girls were taken away.
Why were they taken?
One of those who did escape told the BBC Hausa service the militants had said: “You’re only coming to school for prostitution. Boko [Western education] is haram [forbidden] so what are you doing in school?”
In the past the militants had secretly swapped hostages for Boko Haram members or relatives in prison but this abduction and the blaze of publicity that followed came as the group was growing in strength and capturing territory.
Captives from the villages they were taking over were generally put to work, the boys as fighters while women and girls were often forced to become wives of men in the group.
When were they last seen?
Three videos have been released to date. On 14 August 2016, a Boko Haram tape showed about 50 of the girls and contained a demand for the release of imprisoned militants in exchange for them.
The group also said some girls had been killed or injured in government air strikes.
In April 2016 a video was broadcast by CNN, which appeared to show some of the kidnapped schoolgirls alive. In May 2014, Boko Haram released a video of around 130 girls gathered together reciting the Koran.
In May 2016 one girl was rescued from the Sambisa Forest, a remote area of north-east Nigeria near to the Cameroon border, where many Boko Haram militants have fled.
She was reportedly spotted by a vigilante group in the area, although the circumstances of her rescue are unclear.
She was named locally as Amina Ali Nkeki, aged 19, and may have had a baby. According to one Chibok activist, Amina said six of the schoolgirls had died but the rest were in the forest.
Boko Haram leader Abubakar Shekau has said the girls have converted to Islam and were “married off”.
Last year, three women who claim they were held in the same camps as some of the Chibok girls told the BBC some of them had become fighters – though the testimony has never been verified.
Other rumours or reported sightings of the girls in neighbouring countries have turned out not to be true.
A journalist who is believed to have had contact with the militants, Ahmed Salkida, said in December that sources had told him that the girls were alive. The Nigerian army has since declared Salkida himself a wanted man.
So what is being done to find them?
The US, UK, France have been providing military support and intelligence to help in the search for the girls.
China and Israel have also given assistance.
A boosted regional force with 8,700 troops from Cameroon, Chad, Niger and Nigeria was launched last July.
Thanks to this renewed military impetus, over the last year the militants have been driven from towns and villages to their hideouts in the Sambisa forest – which cannot be accessed by road.
Thousands of people have been freed from Boko Haram captivity, but Amina was the first of the 219 girls to be found.
Is negotiation an option?
President Muhammadu Buhari has said he is prepared to enter into talks if a credible Boko Haram leadership can be identified. The group is said to have recently split over allegiance to so-called Islamic State.
The president said at the end of 2015 that the government did not know the girls’ whereabouts, health or whether they were alive.
He has faced fierce criticism from the Bring Back Our Girls campaigners for the lack of progress, as when he came to power last May he said that Boko Haram could not be defeated without rescuing the Chibok girls.
In January, Mr Buhari promised an investigation into the kidnapping and the government’s response.
Under the previous administration, relations were frosty between the central government and state authorities, and there seems to have been a communication breakdown which hampered an initial rescue effort.
The inquiry may well get to the bottom of this but it will do little with the immediate effort to trace the missing girls.