Two servicemen have died while taking part in a training exercise in the Brecon Beacons in Wales.
A third is in a serious condition and is being treated in hospital.
The Ministry of Defence and Dyfed-Powys Police are working together to investigate the incident yesterday afternoon, which was the hottest day of the year.
"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," said the MoD in a statement.
"The two servicemen's next of kin have been informed. More information will be released in due course but it would be inappropriate to comment further at this stage."
Temperatures reached 29.2C (84.5F) in Usk in nearby Monmouthshire, South Wales yesterday afternoon.
"It is a case of the people succumbing to being affected by the training that they were doing," a source told the Telegraph.
Because of its remoteness and the rugged nature of the terrain, the Brecon Beacons has been used for years for military training exercises.
Infantry regiments train at Sennybridge in the Beacons and the Infantry Battle School is based in Brecon.