Naomi Adamu is quiet. As she talks she rarely makes eye contact, keeping her voice low and steady.

Upon meeting her, few would suspect she survived one of the most harrowing experiences a young woman could go through. But, her timid demeanour belies an extraordinary strength of character.

Naomi, 24 at the time of the attack, was the oldest of more than 270 students from the Chibok Government Secondary School for Girls abducted by the Islamist militant group Boko Haram in April 2014.

Her classmates referred to her as Maman Mu, Our Mother. Her education had been interrupted by health problems as a child.

She is now the main protagonist in a new book on the so-called “Chibok girls”.

‘Christmas message to my father’

Bring Back Our Girls by journalists Joe Parkinson and Drew Hinshaw is based on hundreds of interviews with the abducted students, their families and others connected to their story.

The book explores the girls’ time in captivity in detail, and shows how the social media campaign that made them famous also made it harder to secure their release. Their fame had made them precious commodities, too valuable to let go.

During the three years she spent with Boko Haram, Naomi refused to bow down to pressure to marry one of their fighters, or convert to Islam.

Instead she and another classmate wrote secret diaries in textbooks they were given to write Islamic verses. She kept them hidden in a makeshift pouch tied to her leg.

“We decided that we should write down our stories,” she tells me, “so that if one of us got to escape, we could let people know what happened to us”.

He was sentenced to 20 years in prison for “planning and kidnapping” the students, a statement said. No further details were given about his age, identity, the exact charges, the circumstances of his arrest or what was said in court.

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The first conviction was in February, when Haruna Yahaya, 35, admitted involvement in the abduction and was given two 15-year jail sentences. oth Yahaya and Yusuf were among 1,669 Boko Haram suspects brought before four special civilian courts at a military barracks in Kainji, in Niger state.

A total of 276 girls were abducted from the Government Girls Secondary School in Chibok on April 14, 2014. Fifty-seven escaped in the immediate aftermath. Since then, 107 have either been found or released as part of a government deal with the jihadists. Nigeria has been criticised for arbitrarily arresting civilians in the remote northeast and holding them, often in unsanitary conditions, for years without access to lawyers.

Trials began last October but with the media and public banned from attending, prompting fresh criticism about due process. The media has had to rely on government statements, although it was briefly allowed to observe proceedings in February. The resumption of cases on Monday and Tuesday was not announced beforehand.

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Yusuf was one of 113 people convicted and sentenced on charges including membership of a proscribed organisation, supporting Boko Haram and participating in acts of terrorism. Others sentenced included a Boko Haram leader and a commander’s wife, who was arrested with her two children as they headed to the group’s Sambisa Forest hideout. Some were released because of time already served. Others were handed sentences of up to 30 years with hard labour.

A total of 240 people have now been convicted in connection with the insurgency, according to government statements since October. The latest statement said 111 people were either acquitted or had their cases dropped because of lack of evidence. That takes to 1,054 the number of people who have been freed, which will likely strengthen the argument of critics who say they should not have been detained in the first place.

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She was set to be part of the group of captives who were released on Saturday.

Garba Shehu told local Channels TV that 83 girls were supposed to be freed “but one said: ‘No, I’m happy where I am. I have found a husband.'”

The 82 were released as part of a negotiation brokered by the ICRC.

The government agreed to free an undisclosed number of Boko Haram prisoners in exchange.

The militants are thought to be still holding more than 100 of the 276 taken from Chibok, north-east Nigeria, three years ago. The militant group has also kidnapped thousands of other people during their insurgency in the region.

It is believed that some of those abducted have been married to fighters and had children with them.

Parents of the missing Chibok girls are slowly learning if their daughters are among the freed.

The government published the names of the freed girls on Twitter but in Chibok not everyone has access to the social media site.

Mr Shehu said the government was working to verify their identities, so they can be reunited with their families as soon as possible.

Aisha Yesufu, coordinator of the #BringBackOurGirls campaign group, told AFP news agency, that they were working to match the parents to their daughters.

They were escorted to the reception in the capital Abuja by armed soldiers, after a check-up at a medical centre.

The girls were handed over on Saturday in exchange for Boko Haram suspects after negotiations.

They were from a group of 276 abducted in north-eastern Nigeria in 2014. Before the latest release, about 195 of the girls were still missing.

The number of Boko Haram suspects released by authorities remains undisclosed.

In Abuja, President Buhari’s chief of staff Abba Kyari said: “Welcome our girls. Welcome our sisters. We’re happy to have you back.

“We’re very glad that you are back. That every Nigerian today must be forgetting every other hardship they’re suffering, because it is a very joyous moment. Welcome, welcome, welcome.”

Some of the girls looked tired and confused by all the attention after spending three years in captivity.

Before arriving in Abuja, they were brought by road convoy from a remote area to a military base in Banki near the border with Cameroon.

Our reporter says that many families in Chibok will be rejoicing at this latest news, but more than 100 of the girls taken have yet to be returned.

“This is good news to us. We have been waiting for this day,” Christian pastor Enoch Mark, whose two daughters were among those kidnapped, told Agence France-Presse.

“We hope the remaining girls will soon be released.” It was unclear whether his daughters had been freed.”

A statement from a spokesman for President Buhari earlier said he was deeply grateful to “security agencies, the military, the Government of Switzerland, the International Committee of the Red Cross, and local and international NGOs” for playing a role in the operation.

In a later BBC interview, presidential spokesman Garba Shehu added: “With all of these things together we negotiated over a period of several months, and at the end of it some of their (Boko Haram’s) members were exchanged for the 82 girls.”

‘Two blindfolded men in convoy’- The BBC’s Stephanie Hegarty reports from Lagos

Information about the release began trickling out on Saturday afternoon.

A soldier contacted the BBC to say that more than 80 Chibok girls were being held in an army base near the Cameroon border.

At the same time an official working for an international agency, who assisted with the release, said that several armoured vehicles left Maiduguri – the city at the centre of the Boko Haram insurgency – in a convoy to travel into the “forest” to meet the girls.

He said there were two blindfolded men in the convoy.

The president’s office said that the girls were released in exchange for some Boko Haram suspects held by the authorities – but we haven’t been told how many.

After the Government Girls Secondary School in Chibok, Borno state, was raided in April 2014, more than 50 girls quickly escaped and Boko Haram then freed another 21 last October, after negotiations with the Red Cross.

The campaign for the return of the girls drew the support of then US First Lady Michelle Obama and many Hollywood stars.

Last month, President Buhari said the government remained “in constant touch through negotiations, through local intelligence to secure the release of the remaining girls and other abducted persons unharmed”.

Many of the Chibok girls were Christian, but were encouraged to convert to Islam and to marry their kidnappers during their time in captivity.

Boko Haram has kidnapped thousands of other people during its eight-year insurgency aimed at creating an Islamic caliphate in north-eastern Nigeria.

More than 30,000 others have been killed, the government says, and hundreds of thousands have been forced to flee from their homes.

Boko Haram at a glance:

• Founded in 2002, initially focused on opposing Western-style education – Boko Haram means “Western education is forbidden” in the Hausa language
• Launched military operations in 2009
• Thousands killed, mostly in north-eastern Nigeria, hundreds abducted, including hundreds of schoolgirls
• Seized large area in north-east Nigeria, where it declared a caliphate
• Joined so-called Islamic State, now calls itself IS’s “West African province”
• Regional force has now retaken most of the captured territory
• Group split in August after rival leaders emerged