The mass kidnap of around 270 girls caused international outrage and sparked the hashtag #bringbackourgirls - which was supported by then-First Lady Michelle Obama.
More than 50 escaped soon after being kidnapped, but 220 of the girls, all snatched from the same school, remained in captivity.
A military source said the girls were rescued from Banki near the Cameroon border to Maiduguri, the capital of Borno state where the insurgency started.
When were the Chibok schoolgirls kidnapped?
On the night of 14 April 2014 around 300 schoolgirls were captured at gunpoint by Boko Haram militants.
Residents in Chibok, a remote corner of Nigeria's north-eastern Borno state, had received phone calls warning them of an imminent attack from Boko Haram.
Convoys with heavily armed insurgents were spotted heading towards the town, where a government school had specially opened for students to take final exams.
When the militants struck, around 15 soldiers stationed in Chibok desperately held them off for almost an hour.
But the soldiers were overpowered and at least 300 girls were snatched.
Around 50 escaped by jumping off the back of trucks.
The schoolgirls are among thousands of people abducted by brainwashed terror group over the years.
Why were the schoolgirls captured by Boko Haram?
Boko Haram is a radical Islamist group that opposes the education of women.
The name of the group translates to – Western Education is Forbidden.
But the militant group’s real name is People Committed to the Propagation of the Prophet’s Teachings and Jihad.
Under its version of Sharia law, women should be at home raising children and looking after their husbands – rather than educating themselves.
The group has repeatedly targeted places of learning in deadly attacks and the extremists have targeted girls as young as 12, according to activists.
Many of the kidnapped girls, most of whom were Christians, were forced to marry the Islamic terrorists and became pregnant.
Human rights advocates fear some of the young girls may have been used to carry out suicide bombing attacks.
The militants are also thought to have captured the girls in protest at the security force’s arrest of some of their wives.
What was the #BringBackOurGirls campaign?
The #BringBackOurGirls hashtag took off on Twitter more than a week after the girls were taken.
It was spearheaded by a former federal education minister, Oby Ezekesili, and other Nigerians with large followings on Twitter.
But the campaign soon attracted global attention and in less than three weeks the hashtag had been used over one million times.
Supermodel Cara Delevingne and Michelle Obama added their weight to the campaign, with the former First Lady delivering an impassioned address.
The campaign is credited with drawing worldwide attention to the plight of the girls and heaping pressure on the government to resolve the issue.
More than 20 girls were released in October 2016 in a deal brokered by the International Committee of the Red Cross.
Others have escaped or been rescued, but many are still believed to be in captivity.
Although the Chibok girls are the most high-profile case, Boko Haram has kidnapped thousands of adults and children, many of whose cases have been neglected.
Boko Haram release new video of kidnapped Chibok schoolgirls claiming they do not want to return to their families
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