Inspector Neil Lumsden, head of road policing at Northern Constabulary, said: "As a result of the collision, a participating car has left the forestry track it was travelling on and collided with a group of spectators."
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His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."
The company’s recently appointed Chief Executive Dara Khosrowshahi said the breach was made by two individuals outside the company.