(CNN) -- Two additional suspects have surfaced in the fatal shooting of an American man on a lake that straddles the United States and Mexico last September, Zapata County Sheriff Sigifredo Gonzalez told CNN. David Hartley was allegedly shot on the Mexican side of Falcon Lake. He and his wife, Tiffany, were on personal watercraft on the lake when gunmen opened fire on them, authorities say. The killing remains unsolved. Gonzalez said officials would not release the new suspects' names.