According to AAIB:
Definition of Accident and Serious Incident
Definition of an Accident
"Accident" means an occurrence associated with the operation of an aircraft which might take place between the time any person boards the aircraft with the intention of flight and such time as all persons have disembarked, in which:
I think the main doubt would be: was there anybody on board with the intention of flight?
On a similar case, the
American Airlines B767 uncontained engine failure in Los Angeles on June 2, 2006, NTSB conducted the investigation, even if there was no intention of flight.