Police are continuing to investigate after two soldiers died on a training exercise in the Brecon Beacons on the hottest day of the year.
The soldiers were serving with the Territorial Army in Powys, mid Wales, on Saturday when temperatures exceeded 29.5C.
A third soldier is in a serious condition after the incident, which is thought to have been linked to the hot weather.
Reports suggest the soldiers, understood to be in their late teens or early 20s, were taking part in the selection process for the TA section of the SAS. The Ministry of Defence (MoD) refused to comment.
A military source told the Guardian the exercise the men were taking part in was "intense" and their deaths did not involve firearms or a fall.
"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," a spokesperson said.
The Brecon Beacons is one of several locations British military use as part of their training. It is used because of its relative remoteness.
Members of all four of South Wales' mountain rescue teams said they had been called out to assist when the two servicemen died.
Some 30 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.
Kirsty Williams, leader of the Welsh Liberal Democrats and AM for Brecon and Radnorshire, told the BBC it was a "meltingly hot" day in the area on Saturday.
"It's very very said to think that people that were serving with our armed forces have lost their lives in this way," she added.
Ms Williams said it was a beautiful area which could also be very "treacherous", and the military trained there "for a reason".
Ken Jones, a former member of the special forces who now organises an endurance race in the Brecon Beacons, told the Guardian: "They are out there carrying a third or half of their body weight. When it's hot it's impossible to get rehydrated.