The bodies of four miners have been found five days after an explosion in a West Virginian coal mine last Monday killed 25 others.

Rescue teams found the miners on their fourth search and rescue attempt after earlier efforts were hindered by thick smoke, fire and dangerous gases, including methane, carbon monoxide and hydrogen.

With the death toll at 29, the disaster is the deadliest US mining accident in nearly 40 years.

US President Barack Obama ordered mine safety officials to report next week on the cause of the explosion, the Upper Big Branch mine’s safety record and steps the government can take to improve mine safety enforcement.

A team of federal investigators are expected to start an inspection today to determine the cause of the blast.

The Massey Energy-owned Upper Big Branch mine has seen three fatalities since 1998 and a worse-than-average injury rate in the past ten years, with a 100 safety violations recorded in 2010.