How did Nigeria secure the 21 Chibok girls' release from Boko Haram?

Analysts say the girls' release was part of a bargain with the militants

In the early hours of Thursday morning, 21 of the kidnapped Chibok schoolgirls were released by their Boko Haram captors in the town of Banki close to Nigeria's border with Cameroon, security sources told the BBC.

They were handed over to a team from the International Committee of the Red Cross who provided assistance during the transfer of the girls.

A few hours later this group of young girls, whose kidnap from their school dormitories in the town of Chibok triggered international condemnation more than two years ago, were arriving in the capital Abuja to be received by the country's vice-president.

It was a rare piece of good news in Nigeria's brutal fight against a Boko Haram insurgency that has killed thousands of people and destroyed much of the country's northeast.

• Chibok abductions: What we know

• The town that lost its girls

The girls' release was all the more surprising given that just last month the government - under intense pressure to do more to free the students - announced that negotiations with the Islamist group had broken down.

So why were the negotiations successful this time?

Boko Haram has shown some of those kidnapped on its propaganda videos

There are conflicting reports but one security official told the BBC that four Boko Haram commanders were freed as part of a swap.

The AP news agency also reported that a "handsome ransom" - in the millions of dollars - was paid by the Swiss government on behalf of the Nigerian government.

The Nigerian government has denied any prisoner swap. Vice-President Yemi Osinbajo said: "Absolutely, there was no exchange of any kind."

But, perhaps significantly, he did not rule out exchanges in the future, saying the government would "consider all options available".

Security analysts say it is ludicrous to suggest that Boko Haram handed over the girls while getting nothing in return.

The freeing of the students comes at a time of turmoil in the Islamist militant group. Boko Haram has split in two factions over its links with the so-called Islamic State.

The infighting may have created an opening.

The schoolgirls are being held by former Boko Haram leader Abubakar Shekau, who appears to have lost out in the power struggle.

From a position of weakness, he perhaps concluded he had nothing to lose by releasing some of the girls.

Vice-President Yemi Osinbajo (left) insisted no prisoner swap had taken place

The Nigerian government says that negotiations with Boko Haram will continue to free the 197 students who are still missing.

Sadly for the families it appears that some of the girls have died in captivity.

The mother of the schoolgirl Amina Ali who escaped in May of this year told the BBC some of Amina's classmates were killed in military bombardments.

The respected Daily Trust newspaper also reported that, after more than two years in captivity and after being married off to Boko Haram fighters, some of the girls to do not want to go home.