ABUJA, Nigeria (AP) — Gunmen raided an orphanage in a community in north-central Nigeria and abducted 23 pupils, the authorities said on Monday. Fifteen have since been rescued but eight remain captive.

The attack took place in an "isolated area" of Lokoja, the capital of Kogi State, according to a statement by the state’s commissioner, Kingsley Femi Fanwo. The facility, Dahallukitab Group of Schools, was operating illegally, he said.

The identities of the gunmen are unknown. No group has claimed responsibility for the attack so far.

The statement did not say how old the abducted schoolchildren are but the term ‘pupil’ in Nigeria usually refers to someone in kindergarten or primary school, typically covering ages up to 12.

The north-central region of Nigeria, where Kogi is located, has seen an increase in the activities of gunmen who primarily kidnap people for ransom.

“Upon receiving the report, security agencies, led by the Nigeria Police Force in Kogi State, in collaboration with other security operatives, swiftly mobilized to the scene,” Fanow said.

“Their prompt and coordinated response led to the successful rescue of 15 of the abducted pupils, while intensive operations are ongoing to secure the safe return of the remaining eight victims and apprehend the perpetrators.”

Students’ kidnappings have come to define the insecurity in Africa’s most populous nation, and analysts say it’s often because armed gangs see schools and students as “strategic” targets to draw more attention.

Nigeria, which is Africa’s most populous country, is battling a complex security crisis, especially in the north, where there is a decade-long insurgency.

Among the most prominent Islamic militant groups are Boko Haram and its breakaway faction, known as ISWAP. There is also the IS-linked Lakurawa group operating in communities in the northwestern part of the country that borders Niger.