Methane gas may be bubbling up in a flooded area of West Virginia's Upper Big Branch mine where 29 men died in an April 5 explosion, a federal mine regulator said Wednesday.

Kevin Stricklin, an official with the federal Mine Safety and Health Administration, said several feet of water have kept investigators from searching that area of the Massey Energy mine near where nine of the victims were found.

Stricklin said the bubbling water probably signals the presence of methane. But he cautioned that methane is frequently found seeping from coal seams in underground mines and that the explosion may not have begun there.

Officially, the cause of the explosion hasn't been determined, but MSHA said it suspected methane and coal dust in a preliminary report delivered last April to President Barack Obama.

Investigators have mapped about 90 percent of the mine even though water has kept them from searching two underground areas, said Stricklin, MSHA's administrator of coal mine safety and health.

Both areas are lower than surrounding areas of the mine and haven't been pumped out since the April 5 blast, he said, adding authorities hope to begin draining the larger of the two areas this week in seeking clues to the disaster.

The mine has about 12 miles of underground workings.

The agency has signaled much work remains in the investigation, including about 50 more interviews and testing on electrical equipment.

MSHA said it has interviewed 197 witnesses, collected hundreds of pieces of evidence, taken more than 3,000 photographs and tested 1,800 dust samples.