Abuja - More than 100 schoolgirls kidnapped by Boko Haram were in Nigeria's capital on Thursday, a day after their surprise release that fuelled rumours about how their freedom was won.A total of 104 of the 110 students seized from the school in Dapchi, in the northeastern state of Yobe, on February 19 were dropped off by the Islamist militants on Wednesday morning.The girls - many of them dressed in matching fawn-coloured long hijabs - boarded a Nigerian military transport plane from the Borno state capital Maiduguri to Abuja on Wednesday night.In the capital, they were expected to meet President Muhammadu Buhari as well as undergo medical checks and counselling similar to other former captives released by the group.Five girls are said to have died in the initial stages of the kidnapping, while one girl - the only Christian - is still being held until she converts to Islam.The Dapchi kidnapping revived painful memories in Nigeria of the April 2014 abduction of 219 schoolgirls from the Borno town of Chibok, which caused global outrage.Their unexpected release - and the fact the jihadists were able to come and go unchallenged - dominated newspapers on Thursday, with many questioning whether a deal was done.But Nigeria's Information Minister Lai Mohammed said: "It is not true that we paid (a) ransom... neither was there a prisoner swap to secure their release. "What happened was that the abduction itself was a breach of the ceasefire talks between the insurgents and the government, hence it became a moral burden on the abductors. "Any report that we paid ransom or engaged in prisoner swap is false."Schools shut Security analysts have suggested previous ransom payments from the government as part of prisoner swap deals for some of the Chibok hostages may have been the motive for Dapchi.But Boko Haram's ability to again seize so many students put security at schools in the restive region under the spotlight, after promises of improvements after Chibok.The jihadists have repeatedly targeted schools teaching a so-called Western education in the mainly Muslim region as part of their insurgency that has killed at least 20 000 since 2009.According to the UN children's agency Unicef, more than 2 296 teachers have been killed and some 1 400 schools destroyed in the wider northeast.On Wednesday, the Borno state government announced that all boarding secondary schools outside Maiduguri and the town of Biu would be closed indefinitely with immediate effect.Muhammad Bulama, the Borno commissioner for home affairs, information and culture, said on Wednesday the decision was taken last week after a Boko Haram attack in Rann on March 1.KEEP UPDATED on the latest news from around the continent by subscribing to our FREE newsletter, Hello Africa.FOLLOW News24 Africa on Twitter and Facebook. Three aid workers and eight security personnel were killed in the attack on the remote town near the border with Cameroon, which led to the withdrawal of aid agencies. Schools have previously been shut in the wake of deadly attacks in Buni Yadi, Yobe state, when more than 40 students at a boys' boarding school were killed in a Boko Haram attack on March 1. Aid resumes Bulama said "urgent and immediate measures" were being taken to improve security but the closures are the latest disruption to schooling in an area already hit by low levels of education.At the start of the new academic year in September last year, Unicef said at least 57% of schools in Borno remained closed.It warned the situation threatened to create "a lost generation of children, threatening their and the country's future".Meanwhile, the United Nations said it had resumed humanitarian operations in Rann after a three-week suspension, although overnight stays remain banned until security improves."The authorisation by the United Nations for flights to resume to Rann for aid workers was taken on Monday. The first flights went on Tuesday," said UN spokesperson in Abuja Samantha Newport.UN agencies, and others including Doctors Without Borders, provide life-saving food distribution, medical care, shelter and sanitation programmes to some 80 000 people in Rann.