PORTLAND, Ore. -- Officials launched a criminal investigation Sunday after six sea lions were found shot to death in closed traps on the Columbia River.

The discovery happened less than two weeks after the Oregon and Washington departments of fish and wildlife began a project to trap sea lions so they could save endangered salmon.

Watch FOX 12 Report: Six Sea Lions Shot Dead

The carcasses of four California sea lions and two Steller sea lions were found Sunday about noon. In addition to the Columbia River deaths, three elephant seals were found shot to death Saturday at a breeding ground in central California.

All three species are federally protected by the Marine Mammal Protection Act and Steller sea lions are also protected under the Endangered Species Act, authorities said.

Fish and wildlife officials checked on the sea lions at the Bonneville Dam after noticing the trapping gates were closed. The traps, authorities said, were supposed to remain open.

The six animals found dead Sunday appear to have been shot by somebody on the Washington state side of the Columbia during the night, according to Brian Gorman, regional spokesman for National Marine Fisheries Service, or NOAA Fisheries, in Seattle.

Necropsies were planned for all the animals and the area was being treated as a crime scene by state and federal agencies, Gorman said.

The controversial sea lion traps are in their second week of operation. It's the government's attempt to protect endangered salmon. Two times per week, officials capture California Sea Lions and transfer them to zoos and parks in the U.S. Stellar sea lions, meanwhile, are released back into the water.

Fishermen and American Indian tribes have pushed to protect the salmon and remove the sea lions, by lethal force if necessary, forcing a delicate balancing act by the federal government.

Seven California sea lions were trapped starting April 24 after the 9th U.S. Circuit Court of Appeals approved their capture. One died during a medical inspection before transfer to a Sea World park.

The Humane Society of the United States has gone to court to challenge the authorization, with another hearing set for May 8. Until a judge rules, no animals may be killed.

Visitors to the Columbia River Gorge on Sunday said they were surprised to see the police activity in the area.

"We usually walk and run in the park and watch the seals," said Ralph Lewis, who was visiting the area Sunday. "The next thing I know, it's all blocked off and there are police agencies." I looked and the seals that are usually over there are dead."

Trapping operations will be suspended during the investigation, said Rick Hargrave, Oregon Department of Fish and Wildlife spokesman, who was at the scene.

Investigators will try to determine if there is any link between the animals killed Sunday on the Columbia and the elephant seals killed Saturday near San Simeon on the central California coast, Gorman said.

6 Protected Sea Lions Found Shot Dead