Tests on some of the 25 dolphins which died after Britain's biggest mass stranding in almost three decades showed they had been healthy, rescuers said on Tuesday, as mystery surrounded the beaching in a Cornish river.

The dolphins died after they swam up the Percuil River near Falmouth on Monday morning and were left stranded.

In what one rescuer called a "sea of carnage", marine experts, wildlife volunteers and vets battled all day to save the stricken dolphins.

Seven were saved and led back out to sea.

Investigators are exploring several possible causes, including the theory that the dolphins had become distressed by naval sonar equipment or were frightened by an unexplained explosion several residents reported hearing late on Sunday.

Claims the animals had moved inland to feed on fish, attracted by a large algae bloom caused by a recent bout of hot weather, were dismissed after no food was found in their stomachs.

While confirming it had a "presence" in the area, the Royal Navy said no training or official activity had been conducted there since last Thursday.

A Navy spokesman also said officials had no knowledge of an explosion involving any vessels.

Despite residents contacting two separate conservation groups reporting hearing loud explosions, local police and the coastguard said they had no record of such an incident.

Autopsies carried out by the Institute of Zoology on Tuesday revealed the dolphins had been healthy, with no signs of illness or injury, officials said.

The national co-ordinator from the British Divers Marine Life Rescue charity, Trevor Weeks, told Reuters the tests did not reveal any clues to the incident.

"It was horrible," he said. "It was really distressing for some of the volunteers."

He said it was the largest mass stranding in Britain since 1981 when pilot whales beached on the east coast. More tests are being carried out on the dead dolphins in the coming days.