Four people have died after an aircraft and a helicopter crashed in mid-air over Buckinghamshire.
Two people were killed in each aircraft, Thames Valley Police said.
Police and the Air Accidents Investigation Branch (AAIB) said they have launched a joint investigation to establish the cause of the collision just after midday at Waddesdon Estate, near Aylesbury.
A Wycombe Air Park spokesman said both aircraft came from the airfield.
Supt Rebecca Mears, from Thames Valley Police, said she could not give any details of the identity or the genders of the victims at this stage and her "first priority" was the next of kin.
She said it was "too early to tell" what might have caused the crash.
The AAIB said the plane involved was a Cessna.
Emergency services were called to Upper Winchendon, close to Waddesdon Manor, at 12:06 GMT.
Mitch Missen, an off-duty firefighter, witnessed the crash from his garden.
He said: "I looked up and saw as both collided in mid-air, followed by a large bang and falling debris.
"I rushed in to get my car keys and en route called the emergency services, who I continued to give updates as to its whereabouts.
"Unfortunately, I wasn't able to locate the actual crash site but directed police, fire and ambulance as best I could. Once they were on the scene, I returned home."
Andy Parry, a teacher in Aylesbury, said he was with students at Waddesdon Manor at the time of the crash.
He said they heard a "massive bang" and saw debris in the sky.
There were a number of road closures following the crash but they have since been lifted.
Seven fire vehicles from Aylesbury, Haddenham, Oxfordshire and Berkshire were sent to the scene.
A spokesman for Bucks Fire and Rescue Service said 30 members of staff in fire engines and urban search and rescue vehicles attended.
He added: "I understand it is in a wooded area near the manor."
The Thames Valley air ambulance, two ambulance crews, two ambulance officers and a rapid response vehicle were also sent to the scene.
Hayley O'Keefe, from The Bucks Herald,
said on Twitter a "plume of smoke" could be seen close to Waddesdon Hill after the crash.
The Rev Mary Cruddas from St Mary Magdalene Church, Upper Winchendon, said she had been to the site to see if she could be of any help.
She said: "The area where it happened is off road and difficult to get to."