Nearly 1,000 people have been freed in the past month from Islamic schools in northern Nigeria where they reportedly experienced abuse.
In one such case, police sources said hundreds of men and boys had been freed from a school in Katsina, many of whom had been chained to walls, beaten and sexually abused.
The four raided schools, all in predominantly Muslim northern Nigeria, have much in common. All had managers who portrayed themselves as Islamic clerics teaching students how to be good Muslims.
All the facilities also operated as reform centers to discipline misbehaving children. And all were in poor communities, drawing little attention — until now.
Activists have sought regulation of private Islamic schools for years, but strong traditions have stood in the way.
One such tradition involves a concept among Nigerian Muslims called almajiri.
"Almajiris, according to Islam, means those who migrated to somewhere in search of Islamic knowledge,” said cultural historian Bukar Chabbal. “That is the original conception — one under a strict teacher who teaches them."
Almajiris are usually boys. A parent will send a son to live with an Islamic scholar, known as a mallam, for many years in the hope that the child will receive a sound education in Islamic doctrine.
There are an estimated 10 million almajiris in Nigeria, often seen on the streets begging for food. According to their Islamic teachers, begging helps the students learn humility.
But Chabbal and others say parents are abusing the system, giving their children away to Islamic clerics because they can't afford to raise them themselves.
Sending unruly children to Islamic schools to be disciplined is another traditional practice.
Aliyu Mohammed Tonga, an activist for almajiri children, said that “as I can recall, when we were young, what our parents used to tell us is that someone has been taken to so-and-so person and has been corrected.”
Muslim groups in Nigeria are condemning the raided schools, saying the owners are not real clerics and the schools are not true almajiri schools.
Activists like Aliyu say regulation is necessary, to separate the good from the bad.
"Anybody can come in, even the criminal can come in in disguise and say, 'I'm a mallam,' and he can do what he can do, and that is what happened," Aliyu said.
President Muhammadu Buhari has directed Nigerian police to find abusive so-called Islamic schools and disband them.
Some information for this report came from Reuters.