Part of Bognor Regis town centre has been cordoned off by police due to an incident.
Sussex Police said that just before 1.50am on Sunday (September 22) a 32-year-old man was found in Belmont Street, Bognor, with critical injuries.
At the time he was found, he was taken by air ambulance to hospital in Southampton. His injuries were described as life threatening by police at the time.
The road has been cordoned off while police officers investigate the scene.
Detective Chief Inspector Alex Geldart of the Surrey and Sussex Major Crime Team said: "Our investigation is at an early stage and we are working to establish the full circumstances.
"In particular we are keen to trace the driver of a light or silver coloured car which was seen nearby at the time, who may have information that could assist.
"Anyone who may have witnessed the incident or have other information that could help us is urged to come forward as soon as possible.
"If you saw what happened or have any other information, please contact us by calling 101 or online quoting Operation Fairford.”