According to initial investigations cited by Républicain Lorrain, the man was carrying out maintenance. CCTV showed him entering the tank alone through access at the top, with the side flaps closed, said police.
His colleague did not see him return from the tank and went to look for him, finding him inanimate. It is not known whether the death was related to chemical residues, or discomfort.
An autopsy and forensic investigations are ongoing to determine the cause of death.
