Crews in the eastern U.S. state of West Virginia have begun the task of recovering the bodies of coal miners killed by an underground explosion on Monday.

Federal mine safety officials said Saturday they were not sure how long it would take to recover the 22 bodies still inside the mine.

29 men were killed in the explosion. Two other miners were badly injured by the blast.

On Friday, searchers located the remains of four missing miners. Their families had hoped the men would be found alive in one of the mine's rescue chambers, but officials said the miners were killed by the initial blast.

U.S. President Barack Obama said his administration would take steps necessary to prevent such accidents in the future.

Officials said Friday an investigation into the cause of the explosion could take a year to complete. Authorities suspect a buildup of methane gas led to the explosion. The highly flammable gas is often found in coal mines.

Massey Energy Company, which owns the mine, has a poor record with government mine regulators. Last year alone, it was fined $900,000 for safety violations at the facility.

The death toll of 29 makes the incident the deadliest coal mine disaster in the U.S. since 1907 when 38 miners were killed in the state of Kentucky.

Some information for this report provided by AP and AFP.