The industrial watchdog investigating the incident, Rostekhnadzor, said the Ulyanovskaya coal mine's methane gas detection system had been blocked, allowing the gas to build.

“The automatic system showing methane levels was rendered non-operational ... so that the sensors would not shut down electricity at the mine,” a Rostekhnadzor spokesman said.

The investigators found that managers at various levels at the mine were responsible for giving such orders, to keep the system from sounding a warning and stopping all work at the operation, Australian Associated Press reported.

A nationwide inspection for similar violations of safety rules is now well underway, starting with 60 coal mines in the coal-rich Kemerovo region, where the Ulyanovskaya mine is located.

The mine belongs to the Yuzhkuzbassugol company, in which Russia's second biggest steelmaker, Evraz, holds a 50% stake.