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.
‘Very, very rare’
Mark Jones, from Brecon Mountain Rescue, was on exercise in the area when the thunder storm hit and they went to assist casualties on Corn Du.
“I’ve been in Brecon Mountain Rescue for 30 years and I’ve never known anything like this,” he said.
“It is exceptionally unusual, and for multiple people to be hit in multiple strikes on different peaks – it is very, very rare.”
His team had been in a valley area below Pen y Fan at the time.
“It was very, very loud,” he said.
“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.