* Abduction on Feb. 19 biggest in Nigeria since Chibok in 2014
* Not immediately clear why Boko Haram returned the girls
* 1 girl still held, as many as 5 died in captivity (Adds details)
DAPCHI, Nigeria, March 21 (Reuters) - Boko Haram militants freed scores of kidnapped Nigerian schoolgirls on Wednesday, but some of the released students said five of their schoolmates had died in captivity and another was still being held.
The Islamist fighters, some of then shouting 'God is greatest', drove the students back into the northeast town of Dapchi in a line of trucks in the morning, dropped them off then left, witnesses told Reuters.
"Five among us taken away were dead. One is still with them because she is a Christian," one of the freed girls, Khadija Grema, told Reuters.
Dapchi resident Muhammad Bursari said his niece Hadiza Muhammed, another of the freed girls, had told him the remaining student was still in captivity because she had refused to convert to Islam.
Witnesses said more than 100 of the 110 girls seized on Feb. 19 were returned, though the government issued a statement saying 76 girls had been freed in an "ongoing process".
The kidnapping of the girls aged 11-19 was the biggest mass abduction since Boko Haram took more than 270 schoolgirls from the town of Chibok in 2014 - a case that triggered international outrage.
The Dapchi abduction has piled pressure on President Muhammadu Buhari, who came to power in 2015 promising to crack down on Boko Haram's nine-year-old insurgency and could face the voters again next year. (Reporting by Ola Lanre and Abraham Achirga in Dapchi, Afolabi Sotunde in Abuja, Ardo Hazzad in Bauchi and Ahmed Kingimi in Maiduguri Writing by Paul Carsten Editing by Andrew Heavens)