LONDON, England (CNN) -- Initial post-mortem examinations on some of the 26 dolphins found dead in southwestern England this week fail to explain why the animals swam ashore in Britain's biggest mass stranding of marine animals for nearly 30 years, scientists said Thursday.
Scientists from the Zoological Society of London, which is leading the investigation into the deaths of the dolphins in Cornwall, said they have examined 11 of the animals so far.
"At present, no conclusions can be made on the cause of the stranding," said Rob Deaville, project manager for the UK Cetacean Strandings Investigation Program. "We are currently undertaking detailed post-mortem examinations on all of the stranded dolphins and will also be conducting a number of further tests. Until the investigation is complete, it is not possible to comment with any confidence on any potential reasons for the strandings that took place on Monday."
The scientists plan to examine six more dolphins in London and the remaining nine will be examined at a lab in Polwhele, near where the dolphins were found.
More than 40 common dolphins swam ashore in four spots near Falmouth in Cornwall on Monday morning. Some swam up the Percuil River and some were found near a harbor.
Rescuers worked throughout the day and night to coax the surviving dolphins back out to sea but 26 died.
The animals appeared to be in good physical condition and well-fed, according to Alan Knight of the British Divers Marine Life Rescue charity. He said nothing suggested the dolphins swam ashore because they were unhealthy.
Dolphins sometimes chase shoals of fish onto a sandbank and find themselves stranded when the tide goes out, Knight said, but that doesn't explain why the dolphins in this case were found in four separate places.
The Zoological Society of London could not say when the final post-mortem examinations might be completed.
Common dolphins, which reach lengths of 2.3-2.6 meters (7.5-8.5 feet) and weigh as much as 135 kg (297 lb.), often travel in large groups 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 in Britain was in 1981, when about 50 pilot whales were found beached on the east coast, Marine Life Rescue said.