Hopes were fading for 64 people still missing after an explosion at Russia’s biggest hydroelectric power plant, potentially raising the death toll to more than 70 from one of the worst accidents to beset Russia’s crumbling Soviet-era infrastructure.

Twelve people have been confirmed dead from an explosion that ripped through a turbine room at the Sayano-Shushenskaya plant in Siberia on Monday. But as teams of divers were still scouring the area on Tuesday, officials said it was likely the further 64 people missing had drowned.