Thirty-seven people accused of illegal mining in north-central Nigeria have died in custody, local officials said.

Police have set up an investigation into the deaths, which the Nigerian Security and Civil Defence Corps (NSCDC) blamed on an apparent disease outbreak.

However, an intelligence report shared with the AFP news agency suggested the group died because of overcrowding and poor ventilation inside the detention facility in Niger state.

“We believe they suffocated because there was no ventilation,” the father of one of those detained told the BBC.

Abdullahi Dalhatu said his son had survived because he was near a small window.

“His friend slumped and fainted, and many others also collapsed.”

His son described how detainees began collapsing in the heat of the overcrowded and poorly ventilated cell, his father said.

He said the detainees repeatedly knocked on the door for help but nobody opened it.

The BBC has not independently verified his account.

After news of the deaths spread, angry miners and relatives started attacking public institutions and public transport buses, using stones and sticks before they were dispersed by police who fired tear gas, according to local residents.

A 24-hour curfew has since been declared in the city.

State governor Mohammed Umaru Bago called the deaths “sad and tragic” during a press conference on Thursday.

An NSCDC commander has been detained following the deaths, Bago added.

He declared three days of mourning and postponed the start of campaigning for elections scheduled for January.