Last year, the United Nations Environment Program issued a report that said naval maneuvers and sonar were among the many factors threatening marine mammals such as whales and dolphins.

"Experts from around the country worked on these samples and tests, and the bottom line is that we are not able to reach a definitive cause for the stranding," said Aleta Hohn of the National Oceanic and Atmospheric Administration's Fisheries Service.

In January 2005, 33 pilot whales were stranded on a beach near Oregon Inlet, North Carolina, and a minke whale was found stranded in Corolla, 30 miles (48 km) away. The next day, two dwarf sperm whales were stranded north of Cape Hatteras.

It was impossible to save any of them and the animals that were still alive were euthanized.

"This particular event was both a multi-species stranding and an unusual mortality event," Teri Rowles, a NOAA veterinarian, told reporters in a telephone briefing.

Several of the whales had empty stomachs and one calf was emaciated, but the researchers said that was not unusual in cases of whale strandings.

They found no evidence of unusual overgrowths of algae in the area, which can sometimes release toxins. Some of the whales had heart disease, arthritis and inflammation, and one had a big abscess, but others appeared perfectly healthy.

There were some injuries that may have been caused when the whales stranded, the researchers said.

Hohn said there had been strong winds in the area, which can change currents, and noted North Carolina's gently sloping beaches were perfect for stranding whales.

It was also possible that healthy whales had followed the sick ones onto shore.

'OVERWHELMING LACK OF INFORMATION'

NOAA acoustics Brandon Southall said there had not been an unusual amount of sonar activity in the area. He added there was an "overwhelming lack of information" about how marine animals are affected by sound.

Strandings that have been associated with sonar have taken place near walls or underwater canyons that may channel, amplify or reflect the sound and none of those are found in the North Carolina areas affected, Southall said.

The National Resources Defense Council said the report clearly implicated sonar.

"We sometimes know when sonar has killed marine mammals because it leaves a calling card: bleeding around the brain, holes in the organs, symptoms similar to those seen in human divers with 'the bends,' Michael Jasny, a consultant to NRDC, said in a statement.

"But in other species the signs are less clear. Sonar can cause animals to strand simply by disorienting them ... however, those cases are more difficult to prove."

One of the North Carolina whales did have bleeding around the brain and others had small hemorrhages, but they could have been caused by thrashing around on the beach, Southall said.