A 41-year-old local man, believed to be the driver of the car, was arrested for grievous bodily harm at the time.
This is now being treated as a homicide investigation by Surrey and Sussex Major Crime Team working in conjunction with colleagues in West Sussex Division.
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A number of teenagers, aged 13 to 15, from Portsmouth, have been treated in hospital, after taking tablets believed to be diazepam.
Thursday Evening's Pollen Forecast
Thursday night's weather for the east of the region