Initial post-mortem tests show the animals were well fed and healthy
The Royal Navy was carrying out a survey off Cornwall around the time 26 dolphins died, the Ministry of Defence (MoD) has confirmed.
The admission came after the navy had previously denied having any vessels in the area.
Post-mortem tests gave no clues to the cause of a mass stranding at four creek locations near St Mawes on Monday.
Sonar was used in Falmouth Bay at the time but the MoD said it was "extremely unlikely" to have affected the mammals.
The dolphins died when they became trapped in a number of creeks around the Percuil River.
They appear to have been well fed and there were no obvious signs of disease or poisoning, post-mortem results showed.
Sonar signals from ships are among the possible causes being looked at for the dolphins becoming confused.
An MoD spokesman said: "Royal Navy vessels have not used low frequency sonar anywhere in the South West areas.
"A survey vessel was conducting trials using a high definition, short range side scan sonar for sea bed mapping trials approximately 12 nautical miles off the coast of Falmouth at the time of the incident.
"It is considered extremely unlikely that this operation could have affected the mammals in any way. "
He said there had been no live munitions firings by the navy in the South West area since Friday.
The dolphins became trapped in creeks around the Percuil River
Following the stranding, described as the worst ever seen in the UK, the BBC was contacted by numerous people reporting naval activity in the area.
However, the Royal Navy press office initially told the BBC that none of their ships were in the Falmouth area over the weekend, and the last navy activity in the area had been last Thursday.
The Natural History Museum said its zoologists from the National Whale Stranding Scheme were working with vets from the Zoological Society of London to discover why the dolphins stranded.
It said marine strandings occurred for a variety of reasons, including sickness, disorientation, natural mortality, extreme weather conditions or injury.
Zoologist Liz Evans-Jones said: "Whales and dolphins strand themselves for a number of reasons and we're not sure yet what happened with these dolphins."
It is believed there were about 76 dolphins in the area at the time and at least 40 were helped to safety in deeper waters.
Some of the dead animals found on Monday are believed to have been pregnant. Two of them had to be put down.
Detailed test results on the dolphins are not expected to be back for weeks.
One of the rescuers, Dave Nicoll of the RNLI, said he and his two volunteer crew did their best to help the stranded animals.
He said: "I can't say I've seen such a terrible scene as that which confronted us when we first arrived in the creek with the lifeboat.
"It was horrific, but my two volunteer colleagues, Tom Bird and Joe Sabien, were brilliant.
"They very carefully got into the water to assist the dolphins that were still alive and the results were successful."