But so-called acoustic trauma cannot be ruled out since porpoise remains examined by experts were decomposed, the experts wrote in a report for the National Marine Fisheries Service, a monitoring agency.
"No matter which side you're on, you can interpret the report as supporting your claims," Fisheries Service spokesman Brian Gorman said.
Decomposition of the remains made it "very difficult if not impossible ... to determine a cause of death linked to soft tissue," such as the animal's hearing organs, he said.
The experts found that two of the porpoises died of blunt-force trauma, possibly caused by hitting ships or colliding with other animals. Illness — such as pneumonia and peritonitis — was implicated in the deaths of three porpoises.
No cause of death could be determined for the six other animals.
The investigation followed reports on May 5 of killer whales, porpoises and minke whales suddenly trying to flee waters around Haro Strait, just north of Puget Sound.
The Navy later confirmed that the guided-missile destroyer USS Shoup had been training with mid-range sonar in the area briefly that day.
"We believe this report proves ... what we've said from the beginning," said Rear Adm. Len Hering, commander of the Navy's Northwest operations. "The Shoup did not kill or cause the deaths of those animals."
He said the findings are consistent with the Navy's own report, also released Monday,
Fred Felleman, a spokesman the Orca Conservancy, a private protection group, said agitation displayed by marine mammals on May 5 provided "indisputable behavioral evidence ... of stress. Whether or not this resulted in their death or substantial impact on their hearing or decreased their immunological capacity is difficult to determine."
He called for a halt to use of mid-range sonar in inland waters.
But Herring said the system on the Shoup, the only vessel stationed in the region with mid-range sonar, must still be used occasionally in the interest of national security.
Because of concerns about porpoises, however, the vessel now will get clearance from Pacific Fleet Command and Hering before the system is used.
The effects of sonar on marine life have been debated for years. Federal researchers linked sonar systems to whale deaths in the Bahamas in 2000.