LONDON, England (CNN) -- The British Royal Navy rejected claims Wednesday that one of its vessels using sonar could have caused 26 dolphins to fatally strand themselves in shallow water off the southwest coast of England.
Marine conservation groups have claimed that underwater noise caused by a naval ship could have disturbed the dolphins, leading to their deaths.
More than 40 common dolphins swam ashore in four spots near Falmouth in Cornwall Monday morning. Some swam up the Percuil River; some were found near a harbor.
"This is an unusual event," Alan Knight of the British Divers Marine Life Rescue charity told CNN. "Something spooked the animals because it happened roughly at the same time in the morning.
"We don't know the whole story yet. It could have been the navy carrying out exercises and a loud noise scared them. It could have been a predator, like a whale, that made them come in very, very fast."
The navy admitted that one of its survey vessels had been using sonar to map the seabed approximately 14 miles (23 kilometers) from where the dolphins stranded themselves, The Associated Press reported.
But it said in a statement that it was considered "extremely unlikely" that the operation had affected the mammals.
An initial examination of some of the 26 dolphins found dead in southwest England has brought researchers no closer to knowing what caused the biggest mass stranding of marine animals in Britain in almost 30 years.
The animals appeared to be in good physical condition and well fed, Knight told CNN.
"There was nothing to suggest that they'd gone ashore because they were feeling ill or because they were not healthy," Knight said. "Nothing on the first seven post-mortem gave us any idea what would have made them strand."
Knight said dolphins would sometimes chase shoals of fish on to a sand bank, and find themselves stranded when the tide runs out.
"If all the dolphins had been found in one place, that's a sensible answer," he said. "But to have the same thing happen in four separate locations? I'm not happy with the answer."
Rescuers worked throughout the day and night to coax surviving dolphins back out to sea.
Common dolphins, which reach lengths of 2.3 to 2.6 meters (7.5 - 8.5 feet) and weigh as much as 135 kg (297 lb.) -- often travel in large herds of hundreds or even thousands.
They are extremely active, fast moving, and are noted for riding bow and stern waves of boats, according to the conservation group, American Cetacean Society.
Until now, the biggest stranding was in 1981 when about 50 pilot whales were found beached on the east cost of Britain, Marine Life Rescue said.