A small earthquake or shifting coal plates may have caused the blast that killed at least 44 Siberian coal miners, investigators said Monday.

Three miners remained missing in the latest disaster to strike Russia's hardscrabble coal country, but rescue workers held out little hope of finding them alive.

The blast occurred early Saturday about 1,840 feet down in the Taizhina mine in Osinniki (search), a sprawling community of ramshackle holes and derelict buildings set amid barren hills in western Siberia.

Of 53 men in the mine at the time, only six were rescued, Emergency Situations Ministry spokesman Viktor Beltsov said.

All 44 bodies found have been brought to the surface, Beltsov said. Funerals for some of the victims were expected on Tuesday.

Deputy Prosecutor General Valentin Simuchenkov said the blast happened after the concentration of methane gas in the mine increased by roughly 10-fold in a short period of time.

Investigators were trying to determine what made the methane level increase so quickly, he said, adding that an earthquake or a shifting of coal plates were among the potential causes.

Meanwhile, he said, a criminal investigation has been opened into suspicions of safety violations at the mine, located in the Kuzbass (search) region of western Siberia.

Though relatively new, having opened in 1998, Taizhina (search) was built on the foundation of a closed mine, and the equipment appeared to be run-down.

More than 600 miners are employed at Taizhina, according to the ITAR-Tass news agency said.

Rescue workers dug underground toward the blast site from two sides — Taizhina and an adjacent mine in Osinniki.

Accidents are common in the Russian coal industry, but Saturday's disaster was the deadliest in the Kuzbass since 1997, when a methane blast at a mine in nearby Novokuznetsk killed 67 people.

At Taizhina, about 1,850 miles east of Moscow, one miner was killed in September 2002 when the roof of a ventilation shaft collapsed.