Four people have been killed after a helicopter and two-seater plane collided mid-air near Waddesdon in Buckinghamshire just after midday on Friday.
Air Accident Investigation Branch (AAIB) investigators are at the scene and could be seen carrying what appeared to be wreckage debris in clear plastic bags.
Police have yet to release any details of the victims, saying their priority was finding and informing the next-of-kin.
Emergency services were called to the scene near the village of Waddesdon, which is home to Waddesdon Manor, on Friday afternoon.
There were two people on board both aircraft, Superintendent Rebecca Mears of Thames Valley Police said.
Road closures in the area were lifted on Friday afternoon.
Investigations on the site are expected to continue until Monday.
A spokeswoman for Waddesdon Manor, a large estate near Aylesbury, said the crash had not happened on its grounds.
Waddesdon Manor is managed by the Rothschild Foundation, a family charitable trust, on behalf of the National Trust, who took over ownership in 1957.
RAF Halton, which is around 10 miles away, said no military aircraft had been involved.
A spokesman said: "We can confirm that neither of the aircraft concerned has a connection with either our air force nor the military, and this is as much as we know at this time."