Horse owner Dai Walters and trainer Sam Thomas were among five people injured in a helicopter accident on Tuesday night.
Walters reportedly has more serious injuries than the other following the incident in North Wales.
A spokesperson for the Air Accidents Investigation Branch (AAIB) said: “An accident near Ruthin, North Wales involving a helicopter has been notified to the AAIB.
"An investigation has been launched and a team of inspectors are at the accident site.”
Four people were taken to hospital after the helicopter crashed into the Denbigshire woodlands.
Police were called to the scene near Llandelidan at just after half past five on Tuesday afternoon.
The fire brigade and ambulance service also attended and police said all occupants had been accounted for. It is understood none of their injuries are life threatening.
Chief Inspector David Cust said: "Officers attended alongside colleagues from Welsh Ambulance and North Wales Fire and Rescue as part of a joint emergency response.
"All the occupants have been accounted for, with four people having been taken to hospitals across the area. None of the injuries are thought to be life threatening or life changing at this time."
Resident Huw Howatson, who saw the helicopter crash, told the BBC: "I watched it happen and I was filming the helicopter. I could see it was in trouble and it hit some trees and I had to get out the way.
"A piece of propeller came away. We had to scarper because there was lots of debris, it was just flying everywhere - it was like a movie."
Walters and Thomas had a runner at Warwick on Tuesday afternoon, where Skytastic finished last of four in the novice chase.
