Thirty-seven people accused of illegal mining in north-central Nigeria have died in custody, official say.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 AFP news agency suggested the group had died because of overcrowding and poor ventilation inside the detention facility in Niger state.The deaths have sparked disturbances including attacks on public buildings, and a 24-hour curfew has been put in place in the state capital, Minna, as a result.On Tuesday, a group of young men were arrested for suspected illegal mining near Borgu – a part of Niger state known for small-scale gold mining.Two days later, 37 people were found dead inside the detention centre run by the NSCDC – responsible for protecting critical infrastructure and tackling crimes including illegal mining.NSCDC spokesperson Babawale Afolabi said more than 50 people had been arrested, and they had not been charged because profiling and documentation were still under way before the deaths.The father of one of those detained said relatives went to the NSCDC office seeking their release but were repeatedly told to return later.“Unfortunately, the following day we got the news that our children had died,” he added.Abdullahi Dalhatu told the BBC: “We believe they suffocated because there was no ventilation.”He 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 fainted in the heat of the overcrowded and poorly ventilated cell, his father said.He said detainees repeatedly knocked on the door for help but nobody opened it.The BBC has not independently verified his account.One father told Nigerian media that his son had appeared healthy when relatives took him food the day before his death and called for an independent post-mortem examination.After news of the deaths spread, miners and relatives started attacking public buildings and buses, using stones and sticks before they were dispersed by police who fired tear gas, according to local residents.Niger’s police chief ordered an investigation into “the unfortunate death of suspected illegal miners while in the detention facility of the NSCDC”, spokesman Wasiu Abiodun wrote in a statement.The victim’s bodies have been taken to the general hospital in Minna for examination.The NSCDC had earlier said the detainees were possibly victims of a diphtheria outbreak. But the agency’s national headquarters later said the cause was yet to be established.One relative, whose 17-year-old brother was among those who died, questioned how a disease could have killed so many detainees without apparently affecting their guards.It is not clear yet whether any of the suspects had been charged.Nigeria’s constitution requires anyone detained on suspicion of a criminal offence to be brought before a court within a “reasonable time” – generally within a day where a court is available within 40km (25 miles), or two days in other cases, unless a longer period is considered reasonable.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.The scramble for gold reserves dug up in illegal mines in Niger state has been blamed for fuelling criminal gang violence, according to officials and experts.Much of the gold in the region is trafficked through global supply chains, a 2024 SwissAid report found.