At least 37 people suspected of illegal mining died Thursday while in the custody of the Nigeria Security and Civil Defense Corps in Minna, the capital of Nigeria’s north-central Niger State, Governor Mohammed Umaru Bago told reporters.

Bago described the deaths as “sad” and “tragic” and said a committee of inquiry had been established. He also declared three days of mourning and postponed the start of campaigning for January elections, according to DW.

The suspects had been detained after arrests on Tuesday and Wednesday over alleged illegal mining, said Suberu Siyaka Aniviye, the NSCDC’s head in Niger State. Aniviye said the agency suspected disease was responsible for the deaths, but added that the bodies had been taken to a local hospital for medical examination. He did not identify the suspected disease.

A police investigation has also been launched, and authorities asked relatives of the victims to provide relevant information. However, an intelligence report cited by DW said preliminary information suggested the fatalities may have resulted from overcrowding and inadequate ventilation in a detention facility.

Dauda Shehu, a survivor, told the Associated Press that 65 detainees had been held in a poorly ventilated cell. Shehu said the detainees struggled to breathe, tried to attract guards’ attention by banging on the door and scrambled for fresh air.

A video shared online showed the bodies of dozens of young men, many appearing to be teenagers, while volunteers and relatives gathered nearby. DW reported that Niger State attracts thousands of artisanal miners because of its gold deposits, while officials and experts say illegal mining has also contributed to violence involving criminal gangs known locally as bandits.