At least 37 suspected illegal miners have died while in the custody of the Nigeria Security and Civil Defence Corps, NSCDC, in Minna, Niger State.
The deaths, which occurred in the early hours of Thursday, September 17, have prompted an investigation by the state police command.
The NSCDC said scores of suspected illegal miners were arrested on Tuesday and Wednesday, but several of those detained were later found dead following what it described as a suspected disease outbreak.
However, a preliminary intelligence report reportedly indicated that overcrowding and poor ventilation at the detention facility may have contributed to the deaths.
Niger State Governor, Mohammed Umaru Bago, described the incident as tragic, announcing the establishment of a committee of inquiry to investigate the circumstances surrounding the deaths.
The governor also declared three days of mourning and postponed the commencement of election campaign activities in the state.
The deaths reportedly sparked protests in Minna by groups of miners and sympathisers, with reports of damage to vehicles and other property.
Police spokesman, Wasiu Abiodun, said the state Commissioner of Police had ordered an immediate investigation into the deaths.
Niger State is rich in mineral resources, including gold, tantalite, copper and lithium, attracting thousands of artisanal miners. Illegal mining in parts of the state has also been linked by officials and experts to criminal activities and banditry.