San Diego ? An explosion in the hull of a Navy cruiser during routine maintenance injured six workers on Saturday, one 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,” Luque said. “We don’t know how or why.”