Investigators theorize that the killer of six sea lions on the Columbia River arrived by boat and was familiar with trapping methods, closing the doors of two metal cages before firing a high-powered rifle at the animals within.
The sea lions' carcasses were found Sunday.
Wildlife agents had begun trapping sea lions last month to keep them from eating endangered chinook salmon.
The trapping has been suspended.
American Indian tribes protecting their fisheries and state governments representing commercial and sport fishermen had promoted the sea lion removal.