An investigation into the cause of a light aircraft crash which killed the pilot has proved inconclusive, a report has found.
Prof David Last, from Llanfairfechan, Conwy, died when the Cessna he was flying off the coast of north Wales disappeared on 25 November 2019.
The Air Accidents Investigation Branch (AAIB) said it could not "dismiss" medical incapacitation as a cause.
But the AAIB said it was not possible to say what had caused the crash.
Investigators were able to scan the wreckage of the plane, which hit the water near Puffin Island during the flight from Caernarfon Airport to Llandudno, but were unable to recover it.
They also recreated conditions of the flight to try and work out what went wrong.
"Whilst only a remote visual examination of the aircraft was possible, there was no indication of structural failure and the flight trial conducted indicated an engine failure had not occurred," the report states.
"The flight trial concluded that it was likely the aircraft required an input on the controls in order to enter and maintain the recorded final descent path.
"The pilot had recently been unwell but there was no evidence of medical incapacitation, although this could not be dismissed as a possible cause."
Prof Last, a consultant engineer and expert witness in radio navigation and communications systems, and a professor emeritus at Bangor University, joined the Royal Institute of Navigation in 1972.
Speaking after his disappearance, his family released a statement saying they were "heartbroken".