Federal investigators have found no evidence of a "massive crack" inside a West Virginia coal mine that owner Massey Energy claims could have triggered an April explosion that killed 29 men.

The head of the federal Mine Safety and Health Administration, Joe Main, said Wednesday it's too early to know what caused the worst U.S. mining disaster in decades. The investigation continues.

Massey Energy Co. has suggested a large floor crack near a mining machine could have allowed a surge of explosive methane gas just before the April 5 blast. State mine safety officials, however, have said a preliminary review of data does not indicate a sudden rush of methane.

Main said investigators did not find a giant crack. Massey said it would release photographs of the crack later Wednesday.