The Zasyadko coalmine was hit by a methane explosion early on Sunday, causing a fire to rage through the mine. 457 miners were underground at the time - 357 have since been brought out alive, with 28 subsequently hospitalized.
Prosecutors said the criminal investigation into the cause of the blast would examine whether breaches in health and safety procedures had led to the deaths of the miners, adding that at present they had no "concrete evidence" of any violations.
Rescue teams are continuing their attempts to locate the missing miners, but are being hampered by a new fire which has broken out in the mine.
Earlier Ukrainian Vice-Premier Andriy Klyuyev said, "I can make no comment. The fate of 30 people remains unclear. Temperatures are rising, and a fire has started."
Earlier a trade union official said the chances of finding anyone else alive were slim.
Three days of mourning for the dead have been announced by local authorities.
