At least 52 people were killed in an accident at a coal mine in Siberia on Thursday.
Russian news agencies reported that six rescuers – who went down to save men from a gas leak – were among the dead.
The regional Investigative Committee said three people, including the director of the mine and his deputy, had been arrested on suspicion of violating industrial safety rules.
It said miners had suffocated when a ventilation shaft became filled with gas.
Nearly 300 people were inside at the time, according to authorities, who say only 239 got out. Dozens of people were being treated at a hospital, some for smoke inhalation. Several remain in critical condition.
State television said prosecutors believed there had been a methane explosion.
The Kremlin said President Vladimir Putin had ordered the emergencies minister to fly out to the Kemerovo region to help with the operation.
Kemerovo, over 2,000 miles east of Moscow, has suffered fatal mining accidents for years.