Two people are dead, and two others are in hospital, following separate lightning strikes in the Brecon Beacons in Powys.
Emergency services were called to areas around Pen y Fan in the national park at midday on Sunday.
Two individuals were airlifted to hospital in Merthyr Tydfil with what were described initially as life-threatening injuries.
Two others are also being treated - one at a specialist burns unit in Swansea.
A spokesman for Prince Charles Hospital in Merthyr confirmed on Sunday evening that two of those taken to the hospital had died.
Another individual remains at the hospital, while a fourth person has been transferred to Morriston Hospital in Swansea with burns injuries.
It is understood that the pair taken from the 2,600ft (800m) Cribyn peak in the Beacons included a man in his 50s.
Another of those injured was on Corn Du, south-west of Pen y Fan.
Huw Jones, spokesman for Central Beacons Mountain Rescue, said: "Our prime concern was to get equipment and personnel up on the tops. So I left with the first party from here - taking medical equipment, a defibrillator just in case.
"And just as we were getting up onto the summit, a call came through that there was another casualty on one of the other summits."
He added: "Through the morning, the clouds built up. And off the back of the hot and humid weather that we've had over the last few days, the weather front moving through triggered off the thunder and the lightning.
"And of course, in a mountainous environment, if there's lightning around, the lightning is going to aim for the highest points."
'Very, very loud'
Mark Jones, another of the mountain rescue team members taking part in the exercise, said: "I've been in Brecon Mountain Rescue for 30 years and I've never known anything like this.
"It is exceptionally unusual, and for multiple people to be hit in multiple strikes on different peaks - it is very, very rare."
He continued: "It was very, very loud. We decided to end the exercise - and then the call came in about the lightning strikes."
He said because they were already on the hills, they were able to reach one of the casualties on Corn Du within about 25 minutes.
An RAF Sea King from Chivenor was sent to aid two casualties on Cribyn.
A doctor from the Brecon rescue team then accompanied the two from Cribyn as they were evacuated to hospital by air.
Three other mountain rescue teams - Longtown, Central Beacons and Western Beacons - were also called out.