Thirty-seven suspected illegal miners who died in the custody of a Nigerian security agency last month suffocated in an overcrowded cell, a government investigation panel said Tuesday.

The deaths have sparked outrage and protests in Nigeria and turned a spotlight on the detention system in the country.

The suspects were arrested by officers of the Nigeria Security and Civil Defense Corps, or NSCDC, a paramilitary agency, and accused of illegal mining in Niger state in September.

The agency initially blamed the deaths on a disease outbreak in the cell, a claim survivors rejected.

Jonathan Kure, the head of the committee set up by the Ministry of Interior to investigate the deaths, said that about 50 male detainees were confined in a space measuring about 20 square meters (215 square feet).

The deaths were caused by “severe overcrowding, unlawful and unsuitable detention conditions, failure of duty of care, disregard of repeated distress warnings, inadequate supervision and wider systemic deficiencies,”…