An explosion in the hull of a Navy cruiser during routine maintenance injured six workers on Saturday, two of them critically, authorities said.

Subcontractors from the National Steel and Shipbuilding Co. were working in the fuel tank of the USS Lake Champlain as it sat in dry dock when the explosion occurred just after noon, said San Diego Fire Department spokesman Maurice Luque.

"Apparently it was caused by flammable gas that ignited," said Maurice Luque of the San Diego Fire Department. "We don't know how or why."

Two of the workers were taken to UC San Diego Medical Center with "major trauma burns" that Luque said were life-threatening.

Two others had moderate injuries and were taken to the hospital, Luque said, and two more were treated at the scene and released.