A passenger was killed and four other people injured when a car crashed into a brick wall after a police pursuit.
The Independent Police Complaints Commission (IPCC) has begun an investigation into the crash in Brighton on Saturday.
IPCC commissioner Tom Davies said officers in a patrol car saw a Mazda with five young people in it, allegedly being driven erratically.
The driver was arrested by police after the crash in the early hours, he said.
He said the officers in the patrol car signalled to the Mazda for it to stop.
'Thrown from vehicle'
But the driver allegedly did not respond and, after a pursuit, the vehicle crashed into a wall, killing one passenger and leaving another critically injured.
The five young people - three young men and two young women - were all taken to the Royal Sussex County Hospital.
One has since been discharged.
Fire crews said at least two people in the car were thrown out of the vehicle because of the "ferocity" of the crash.
The person who died has not yet been identified, and will not be named until next week.
Mr Davies said: "My condolences go to the family and friends of the dead man.
"This must have been a traumatic incident for everybody involved, including the members of the emergency services who attended.
"The IPCC investigation will be independent and proportionate."
He said the IPCC deployed two investigators to the scene of the crash, which happened at about 0100 GMT in Wilson Avenue.