During the night of April 14, 2014, one of the world's deadliest militant groups, Boko Haram, carried out an attack on a girl's boarding school in Borno State's Chibok town in northeastern Nigeria.
The militants raided the dormitories and 276 schoolgirls were forced into vans by Boko Haram militants.
Recently, in October 2016, 21 Chibok girls were released by Boko Haram after negotiations with the Nigerian government. But there are still 195 schoolgirls who remain in captivity, waiting to be freed.
In a message to mark the mass kidnapping, Nigeria's President Muhammadu Buhari said on Sunday that his government was committed to ensuring the schoolgirls are reunited with their loved ones.
"We are hopeful that many more will still return," Buhari said. "It is a goal we remain steadfastly committed to."
Negotiations between the Nigerian government and Boko Haram to gain the release of the girls began in July 2015, but they were derailed, mainly due to a split within the ranks of Boko Haram.
With then-president Goodluck Jonathan's failure to react to the kidnapping, the national outrage toward the government brought international attention to the militant outfit.
What has been done to combat Boko Haram?
President Muhammadu Buhari, who came to power in 2015, has made the fight against terrorism a top priority for his administration and has vowed to defeat the militants.
However, he has been facing criticism for not securing the release of the kidnapped schoolgirls.
Last July, a multi-national joint task force (MNJTF) was created, comprising of troops from countries of the Lake Chad region that include Nigeria, Chad, Cameroon, Niger and Benin.
The task force managed to push Boko Haram and associated militant groups out of towns they had previously captured.
But the outlawed group continues its insurgency. As a new strategy, the outfit decided to carry out deadly suicide attacks instead of capturing and holding a area.
In order to launch such attacks, the group uses children as suicide bombers.
During the past three years, the group has kidnapped more than 10,000 boys and trained them in camps, situated in abandoned villages and forests, according to government officials in Nigeria, its neighbouring country Cameroon, and Human Rights Watch, and a New York-based advocacy group.
As of May 2016, the militant group has used 113 females as suicide bombers, according to The Long War Journal.
What is Boko Haram?
Jama'atu Ahlis Sunnah Lidda'a Wati Wal Jihad, better known by the more colloquial Hausa language term Boko Haram, opposes "Western-style education" taught in public schools in Nigeria.
Boko Haram translates to "Western education is forbidden," an idea which originated from the teachings of a Nigerian cleric, Muhammad Yusuf, who established Boko Haram in 2002. He also called for the removal of any form of secular authority.
Eventually, things came to a head in 2009 when a clash between Yusuf's followers and the police led to a bloody showdown which lasted several days.
The DAESH-affiliated group continues to devastate the lives of millions of people in northern Nigeria and the entire Lake Chad region.
Since 2009, the group has killed nearly 20,000 people and displaced more than 2.5 million others.
The most brutal massacre perpetrated by Boko Haram to date took place in the north-eastern town of Baga, which is home to 300,000 residents. The multi-day attack left at least 2,000 dead in January 2015.
"We just can't forget the 195 of them that are still there," said Aisha Yesufu, a representative of the Bring Back Our Girls movement.
"We have to look and bring them back home."