Police are continuing to probe the deaths of two servicemen who died "yomping" in the heart of some of Wales' most rugged terrain, during a military training exercise on the hottest day of the year.
A third serviceman was last night in a serious condition in hospital.
The three were part of a group training in the Brecon Beacons on Saturday when temperatures topped 29.5C (85.1F).
Soaring temperatures may have been to blame for their deaths, military sources say.
Yomping is a Royal Marines term for long-distance marching over difficult terrain, carrying heavy weights.
The soldiers are understood to have been taking part in SAS selection, where many exercises are undertaken as individuals. Soldiers normal operate within the "buddy buddy" system, where servicemen are put in pairs to raise the alarm if something happens to their companion.
The Ministry of Defence and police said they were both investigating the incidents, which happened near the Storey Arms Centre.
A spokesman said: "The MoD can confirm that it is working with Dyfed Powys Police to investigate an incident during a training exercise on the Brecon Beacons on Saturday in which two members of military personnel died.
"The two servicemen's next of kin have been informed."
The Brecon Beacons is one of several locations British military use as part of their training.
Its rugged and sprawling terrain helps prepare soldiers physically and mentally for warfare as well as put their logistic skills to the test, making it an ideal area for elite forces personnel like the SAS.
However the Beacons' jagged topography can prove dangerous even to the most hardened and physically fit.
Earlier this year an army captain was found dead on a snow-covered Corn Du mountain. It was thought that Rob Carnegie had been taking part in a gruelling 17-40 mile march in freezing conditions in the Brecon Beacons as part of a selection process for the special forces regiment, when he collapsed and died.
However, this time investigators are examining whether hot temperatures played a major factor.
A source said: "It is a case of the people succumbing to being affected by the training that they were doing."
The Army's website said the Brecon Beacons were used because they are so demanding and prepare soldiers for the "extraordinary things" they have to do on deployment.
The website says: "Training for high-intensity, light-role war fighting is the way soldiers and officers are prepared for any operational situation they may face - conventional war, counter insurgency, security sector reform, peacekeeping or supporting civil authorities.
"This ensures that the training is as close to current operations and pre-deployment training as possible, whilst maintaining the ability to train for high intensity war fighting."
Major Alan Davies, who was involved in contingency planning during the first Gulf War, told the BBC that the Beacons is one the most challenging terrains military personnel can encounter.
"On one end of the spectrum you have cadets being taken for mountain walking and at the other end of the spectrum the SAS use it," he said.
He added that the three men may have been carrying very heavy equipment and working to a deadline, which meant they would have been pushing themselves very hard.
News of the deaths has been met with shock in the nearby town of Brecon, which is home to The Infantry Battle School
Brecon mayor and Powys county councillor Matthew Dorrance said: "It's incredibly sad for the friends and family of the people who have lost their lives and our thoughts are with the person who is injured.
"In one way we've been blessed with the weather but for people working in this heat, they're tough conditions."
Dorrance added that locals regularly saw troops training in the area.
"We're proud of our links with the military in the town," he said.
A Dyfed Powys Police spokesman said: "The Ministry of Defence are fully co-operating with our inquiries.
"The next of kin and HM coroner have both been informed. "
David Davies, the Conservative MP for Monmouth, told BBC News: "It's a terrible tragedy, and I'm sure the Army will hold a full investigation and be doing absolutely everything to find out what went wrong, and prevent that from happening again."
A former member of the Territorial Army, he added: "Nobody should jump to any conclusions here about what may have caused this, but obviously we all know that, for example, people who take part in marathons run a small risk of dying of heat stroke and all sorts of other things, and the British Army does train its soldiers very very hard indeed.
"The training is there for a purpose, and it is arduous training at any level, and sometimes things tragically go wrong."
Members of all four of South Wales' mountain rescue teams said they had been called out to assist when the two servicemen died.
Thirty members of Central Beacons, Brecon, Western Beacons and Abergavenny-based Longtown Mountain Rescue Teams joined the operation near Pen y Fan, which is the highest mountain in south Wales.
Mark Moran, from Central Beacons MRT, paid tribute to its members who took part in the rescue operation.
"They are all volunteers, who are highly trained and dedicated" he said.
"We were working alongside military personnel who remained extremely calm and professional during this tragic incident.
"Our thoughts are now with the families of those involved."