Marine experts believe 26 dolphins who died in Cornwall became stranded after being frightened by an underwater disturbance.
A number of theories have been forwarded to explain the deaths, one of the worst mass strandings ever in the UK.
It had been thought the striped dolphins came inland to chase fish and then became beached along the Percuil river, near Falmouth.
But Tony Woodley, spokesman for British Divers Marine Life Rescue (BDMLR), said: "Very few had anything in their stomachs.
"That adds weight to the theory that they were scared ashore.
"It is another unusual factor in this extremely unusual stranding."
Attention has now switched to the Royal Navy, which was carrying out exercises in Falmouth Bay with ships and a submarine.
The Ministry of Defence said a survey vessel had been using low power, high frequency, short range equipment to map the sea bed about 12 nautical miles off the coast of Falmouth at the time.
A spokesman said: "It is considered extremely unlikely that this operation could have affected the mammals in any way."
He also confirmed that live firing exercises had been carried out off Eddystone Light, south of Plymouth, on Friday.
But the dolphins were found dead early on Monday morning, 60 nautical miles away.
A Royal Navy spokesman added: "There has been no evidence of any of our vessels' sonar ever playing a part in beaching incidents like this.
"The Royal Navy takes take its environmental responsibilities very seriously and always carry out environmental assessments before putting energy in the water."
He said no vessel in the South West had been using low frequency sonar at the time and the submarine's sonar had not been transmitting.
Alan Knight, chairman of BDMLR, said: "I have never heard of anything like this, certainly not in the UK and possibly not anywhere.
"My own personal conclusion is that there was some sort of disturbance that has caused the animals to panic."
Another theory was that a pod of 15 dolphins became stranded, issued distress calls which were picked up by others who followed the signal and also became stuck.
The Natural History Museum said its zoologists from the National Whale Stranding Scheme were working with vets from the Zoological Society of London to unravel the mystery.
It said marine strandings occurred for a number of reasons, including sickness, disorientation, natural mortality, extreme weather conditions or injury.
Sky News