Two servicemen who died during a military training exercise on the hottest day of the year were entering the selection process for the Territorial Army's section of the SAS, according to reports.
A third serviceman remains in a serious condition in hospital after the incident in Powys on Saturday.
The trio formed part of a group training across some of Wales' most rugged terrain in the Brecon Beacons on Saturday when temperatures hit 29.5C, making it the hottest day of the year.
Military sources have said that the soaring heat may have contributed to their deaths. The three men could have been carrying very heavy equipment and training to a deadline, which meant they would have been "pushing themselves very hard", Major Alan Davies said.
However, the MoD has refused to confirm reports that the three men were aspiring to join the reservists' branch of the SAS. A spokesman added that there are no plans to change “routine exercises” in light of the incident.
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 the snow-covered Corn Du mountain. It was thought that Rob Carnegie had been taking part in a gruelling 17 to 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 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.
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' mountain rescue teams were called out to attend the scene 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.
Live ammunition was not thought to be involved in the incident. Police and the Ministry of Defence (MoD) are continuing to investigate their deaths.
Additional reporting by PA