A woman was fatally struck by a police van which had its blue lights on, investigators have said.
Officers had been looking for the woman, who was reported missing, when their marked van hit her in Hethel, near Norwich.
The woman, in her 50s, was hit at around 10.20pm on Thursday, July 27.
She was taken to Addenbrooke's Hospital in Cambridge by the East Anglian Air Ambulance (EAAA) piloted by the Duke of Cambridge on his final shift as an air ambulance pilot.
MORE: Missing woman hit by police van in Hethel has died
She died two days later, on July 29.
The Independent Police Complaints Commission (IPCC) has begun an investigation into the incident and a spokesman said early indications are that the police van had its blue lights on at the time of the collision.
IPCC operations manager Adam Stacey said: 'This is a tragic case and my thoughts are with the woman's family and friends at this time.
'The IPCC has now begun an investigation to determine the circumstances that led to her death and the actions of Norfolk police officers that night.'
Officers were responding to a 999 call 'relating to concerns for the safety' of the woman, and an IPCC spokesman said these concerns were raised by the woman's family.
Investigators have met with the woman's family and will liaise with the coroner ahead of an inquest.
Flowers were left at the scene of the crash after it was announced the woman had died.
The road remained shut for nearly 17 hours after the crash before it reopened just after 3pm on Friday, July 28.
William announced in January that he would be ending his career with EAAA and has clocked up more than two years of flying medical crews to emergencies.
The Duke was based at Cambridge Airport as part of a team including specialist doctors, critical care paramedics and pilots providing emergency medical services across Bedfordshire, Cambridgeshire, Norfolk and Suffolk.
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