Two servicemen who died during a military training exercise on the hottest day of the year were taking part in the selection process for the Territorial Army's section of the SAS, according to reports.
Police and the Ministry of Defence (MoD) are continuing to investigate their deaths, while a third serviceman remains in a serious condition in hospital.
The trio were part of a group training in the Brecon Beacons, some of Wales's most rugged terrain, on Saturday when the mercury hit 29.5C (85.1F).
Soaring temperatures may have been to blame for their deaths, military sources said, and it is understood live ammunition was not involved in the incident.
The MoD has refused to confirm reports that the three were members of the TA aspiring to join the reservists' branch of the SAS.
And a spokesman said there are no plans to change "routine exercises" in light of the incident.
An unnamed Army officer told the Daily Mail: "The troops are desperate to prove they are up to joining the SAS and push themselves harder and harder. They are encouraged to by the instructors who are unforgiving.
"There is going to be a huge inquiry into what happened because, on the face of it, these lads look like they've been marched to death.
"More should be done to ensure these needless tragedies don't occur."
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.
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 was 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."
Members of all four of South Wales's mountain rescue teams were 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.