A man died after the ambulance he was being treated in, being driven by a police officer, left the road in poor weather conditions in East Sussex.
Daniel Martin, 31, of Horam, had earlier received life-threatening injuries when his Mini collided with a highways lorry.
Because of the severity of his injuries, both ambulance crew members were needed to deal with Mr Martin.
However, the ambulance then left the road and hit a bush in Ninfield.
A man driving the highways vehicle was arrested on suspicion of dangerous driving and later freed on bail.
Sussex Police said the 36-year-old, from St Leonards-on-Sea, was bailed to 17 February, pending further inquiries.
A post-mortem examination is due to take place on Wednesday.
The first accident happened just before 1430 GMT on Friday when Mr Martin, who was driving a Mini, was in collision with the highways vehicle on the A271 Boreham Street, near Herstmonceux.
A police officer then drove the ambulance as two paramedics tried to treat him, as he had suffered life-threatening injuries.
Twenty minutes later the vehicle crashed into a bush in Standard Hill in poor weather. An ambulance crew member was later treated in hospital for minor injuries.
The Sussex Police helicopter and the Kent Air Ambulance attended the scene of the second accident but Mr Martin died there.
A police spokesman said it was an "unusual occurrence" but was normal practice for police officers to take the wheel of an ambulance in a medical emergency.
Det Ch Insp Mike Ashcroft, of Sussex Police, said: "This is a complex series of interlinked events, which happened during adverse weather conditions.
"A detailed investigation involving forensic collision investigators is continuing against a backdrop of worsening weather."
Sussex Police subsequently contacted the Independent Police Complaints Commission, which is expected to decide on Monday whether to investigate.
The flatbed highways vehicle was carrying road equipment and operated by a contractor on behalf of East Sussex County Council.