UK investigators have so far been unable to discover why a Leonardo Helicopters AW169 lost control and crashed shortly after take-off, killing all five on board.
In its initial report into the 27 October incident, the UK Air Accidents Investigation Branch says that the "cause of the apparent loss of yaw control has yet to be determined".
"Investigation of the tail rotor control system is being carried out as a priority," it says.
The report notes that the manufacturer has issued an alert service bulletin, subsequently mandated by European safety regulators, calling for precautionary inspections of tail rotor control assemblies on the global AW169 fleet.
Issued on 14 November, the AAIB report discloses no flight abnormalities during the initial part of the medium-twin's vertical departure from Leicester City Football Club's King Power stadium.
However, the report says that, as the rotorcaft climbed out the stadium on a northerly heading, the "helicopter entered an increasing right yaw contrary to the pilot's left pedal command" shortly after passing 320ft. "The helicopter reached a radio height of approximately 430ft before descending with a high rotation rate."
The rotorcraft crashed into the ground in an approximately upright position with its landing-gear retracted, the AAIB says, before rolling onto its side.
An "intense" post-impact fire prevented any rescue of those on board, who included Leicester City owner Vichai Srivaddhanaprabha.
The AAIB records that the weather was clear at the time of the accident, with good visibility and wind from the northwest of 25-30kt (46-55km/h) at 1,000ft.