More than 50 people were reported to have died on Thursday after smoke filled a Siberian coal mine and a rescue effort ended in tragedy.
Senior managers at the mine in Russia's Kemerovo region had been detained for suspected safety violations, following the latest deadly accident to hit the country's vast mining industry.
Russian news agencies quoted local authorities as saying 52 people were dead, including miners and six rescuers who were part of an aborted search operation.
"According to preliminary information, no one is left alive in the mine," state news agency Tass quoted a source in the local emergency services as saying.
The Listvyazhnaya mine, near the town of Belovo in the Kemerovo mining region, filled with smoke in the early hours of Thursday with 285 people inside.
Most were able to get out, but officials said 35 miners were unaccounted for.
It was not immediately clear what caused the accident, although some Russian media reported dust in a ventilation shaft caught fire, causing the mine to fill with smoke.
The RIA Novosti news agency quoted prosecutors as saying a spark may have set off a methane explosion.
A search and rescue operation was launched but called off several hours later after officials warned of an explosion.
There were reports of rescuers running low on oxygen and news agencies said the bodies of six of them were found.
Private news agency Interfax quoted a local official as saying they had suffocated.
Russia's Investigative Committee, which handles major cases, opened a criminal probe and said three people had been detained including the 47-year-old director of the mine and his 59-year-old first deputy.
"During the initial investigation, it was established the detainees violated industrial safety requirements," the committee said.
Regional authorities said nearly 40 people were hospitalised.
The governor declared a three-day mourning period in the region beginning Friday.
Earlier in the day residents of the village of Gramoteino next to the mine gathered in snowfall and sub-zero temperatures outside the entrance to the site, which was closed off with a boom gate, as ambulances passed in and out.
There was no panic but the crowd was tense and refused to speak to journalists.
"This is a great tragedy," President Vladimir Putin said during talks with Serbian leader Aleksandar Vucic in Sochi.
The Listvyazhnaya mine was set up in 1956 and is owned by the SDS-Ugol company based in the city of Kemerovo.
A methane blast killed 13 people at the mine in 2004 and another explosion at the site killed five people in 1981.
Mining accidents are fairly common in Russia and across the former Soviet Union as a result of poor safety standards, a lack of oversight of working conditions and ageing Soviet-era equipment.
One of the deadliest mining accidents in Russia in recent years occurred at the Raspadskaya mine in Siberia, Russia's largest coal mine, in the summer of 2010, killing 91 people and leaving more than 100 injured.
The incident was the result of a methane explosion when more than 300 miners were inside. A second explosion then trapped a group of rescuers.
In 2007, 110 people died after a gas explosion at the Ulyanovskaya mine, in the same region.
On top of the deadly accidents, activists have drawn attention to the environmental practices of mines in Russia, one of the world's largest mineral producers.