Soaring temperatures may have caused the deaths of two servicemen while training in some of Wales' most rugged terrain, military sources say.
The men had been on an exercise in the Brecon Beacons yesterday - where temperatures topped 29.5C (85.1F) - before they died.
A third serviceman was taken to hospital and remains in a serious condition.
The Ministry of Defence and police said they were both investigating the incidents, which happened near the Storey Arms Centre.
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."
Emotional tributes have been paid to a third army reserve soldier who died from injuries sustained during SAS selection training.
One of two Army reservists killed in a military exercise in the Brecon Beacons on Saturday has been named as Lance Corporal Craig Roberts.