The scientists began work Tuesday in the dissection room at the National Marine Mammal Lab, part of the National Marine Fisheries Service's regional headquarters on Lake Washington.
The exams will help determine whether mid-range sonar testing May 5 by the guided-missile destroyer USS Shoup was a factor in any of the deaths. Environmental groups say the sonar can interfere with marine mammals' communication systems.
In all, 13 dead porpoises were found beached or floating between May 2 and May 20 - eight of them on or after May 5. Two carcasses floated away, so scientists are working with the nine carcasses and two heads recovered.
Springtime strandings are common, federal scientists note.
"At this point, there is no evidence any of these deaths were connected to the Shoup except for the coincidence in time," said Brent Norberg, standing coordinator for the fisheries service.
The vessel conducted the tests in Haro Strait, which separates Washington's San Juan Islands and Canada's Vancouver Island.
The porpoises, about 4 feet long and weighing up to 100 pounds, have been frozen since they were retrieved from Olympic Peninsula beaches. They were trucked to a private radiology lab Sunday for CT scans and have been thawing since in big, water-filled pans.
Fourteen scientists enlisted from universities, government and military offices as far afield as Massachusetts, North Carolina, California and Canada "collectively are probably the most knowledgeable in the world on this subject," said the service's Regional Administrator Bob Lohn.
Performing the necropsies, the equivalent of autopsies in humans, is expected to take at least three days.
The scientists will not know when or where each animal was found, service spokesman Brian Gorman said, so as not to let that influence their conclusions. Tissue and fluid samples will be sent to labs around the country for tests.
In 2001, preliminary tests by the Navy and the fisheries service determined that sonar tests in the Bahamas likely caused the deaths of at least a half dozen beaked whales whose carcasses were recovered.
A coalition of environmental groups led by the Natural Resources Defense Council sued the Navy and the fisheries service in federal court last summer to block the use of low-frequency sonar to identify enemy submarines.
The Defense Department is asking Congress for expanded exemption from the Marine Mammal Protection Act and other environmental laws to continue using the sonar system.
By Peggy Andersen