Two Air Training Corps cadets and their RAF instructors have been killed in a mid-air crash between two light aircraft near the south Wales coast.
The cadets, who were teenage girls, were cousins. They died after a collision near the seaside resort of Porthcawl just before 1100 GMT.
The Grob planes had been involved in 20-minute "air experience" flights from RAF St Athan, Vale of Glamorgan.
The wreckage was spread over about half a mile near a nature reserve.
Three separate inquiries are under way into the crash.
Group Captain Andrew Naismith describes the loss to the "RAF family" of the cadets and their tutors to Penny Roberts.
At least one of the twin-seater planes was on fire when emergency services reached the scene.
One eyewitness told the BBC she saw two planes hit each other, before she heard an explosion.
Reena Callingham told the BBC News website she was putting out her washing when she saw two planes flying close together.
"I just couldn't believe it. At first there wasn't an explosion straight away but then there was. They just went down," she said.
Police said wreckage was strewn across quite a wide area and officers were receiving reports from members of public coming across the debris.
Flight lieutenant Alastair Hawes said the planes were on "air experience" flights from RAF St Athan, about 20 miles away.
Such flights usually feature an RAF instructor alongside someone, usually a young person or cadet, who is interested in a military career.
The crash scene was close to the M4 motorway, near a nature reserve
The aircraft involved were Grob Tutors, twin-seater light aircraft with propellers, popular with flying training.
Although the aircraft were based at St Athan, their "parent" headquarters is the number one Elementary Flying Training School at RAF Cranwell, Lincolnshire.
Andy Naismith, group captain in charge of flight training at Cranwell, said a service inquiry was being run in parallel with the police and air accident investigation.
He said: "We would like to learn what lessons we can from this tragic incident."
He added: "Four members of the RAF family have tragically lost their lives following an accident which took place near Porthcawl just before lunchtime today.
"Those who died were two members of the RAF and two Air Training Corps cadets. "
Grp Cpt Naismith said next of kin have been informed but the families want more time before they are named "to let them absorb and come to terms with this devastating news".
An RAF Grob training aircraft - photo: Air Team Images
"I would like to hereby thank all members of the emergency services for their rapid and caring response and once again ask for the media to exercise restraint at this most difficult time for the families," he added.
Supt Tim Jones, of South Wales Police, said: "A major incident room has been set up at police headquarters in Bridgend and a response is being co-ordinated from there."
He said it was a complex response because of the nature of the scenes they were managing and that it would be a joint investigation.
"Clearly, the scene is expansive and we will be needing to protect the scene tonight to maintain the integrity of the investigation and we would appeal to members of the public to avoid the area tonight," he said.
Aerial footage of the air crash scene near Porthcawl
Rod Hammerton, head of operations for South Wales Fire and Rescue Service, said some 90 firefighters were initially sent to the scene of the crash, near Kenfig nature reserve.
He said: "Twenty-four appliances and 90 firefighters were despatched, as well as chemical units and search and rescue. During the incident most resources were not used.
"We would like to extend our sympathy to those who have and will be touched by this, and would like to thank the other emergency services."
A number of helicopters were used during the incident to gather evidence and mountain rescue teams were on stand-by.
The inquiries will be undertaken by police, Ministry of Defence and the Air Accident Investigation Branch (AAIB).
A casualty bureau number 0800 096 1011 has been set up for members of the public to contact if they are concerned relatives may be involved.
Anyone who witnessed the crash or has information is asked to contact South Wales Police on 01656 306102 or 01656 306103.
Police and salvage teams along the dunes, looking for aircraft wreckage
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