A car in which five teenagers died had been stolen, the BBC has learned.
Matthew Baker said what happened left him "numb and shocked"
The Independent Police Complaints Commission (IPCC) is investigating because police were "involved" with the vehicle just before the crash.
Matthew Baker, from St Leonards, East Sussex, told BBC South East Today it was his blue Metro which crashed in the town in the early hours of Saturday.
All the victims were aged between 14 and 17. One was named as Daniel Carwardine, aged 16.
His father, Lee, laid flowers at the scene of the crash.
"It's just like a nightmare," said Mr Carwardine.
"I just want someone to wake me up. And it's not just me, it's four other families as well.
A card left with flowers said: "To our lovely son Daniel. We will always love you, thank you for being our son for such a brief time. Mum and Dad."
Sussex Ambulance Service said all five teenagers - believed to be boys and girls - were pronounced dead at the scene.
The accident happened at 0142 BST on Battle Road.
Police officers in a marked vehicle made contact with the Metro just two minutes earlier after receiving a report from a member of the public about it.
Mr Baker said his father noticed his car had been taken at about 0630 BST.
"I rang the police and reported it," he said.
"Within about half an hour I got a call back from an officer who wanted to come round and take a statement.
Lee Carwardine was one of several grieving relatives leaving flowers
"About an hour later he came round and he informed me that my car had been found and it was in an accident and there had been five fatalities, which was quite shocking obviously."
The IPCC refused to confirm whether the car was stolen, but local people believed it had been.
Michelle O'Callaghan, who runs a video store in Battle Road, said: "Local teenagers have been coming in all day saying the car was stolen and that the police were chasing them.
"It's a real tragedy and everyone is shocked. "
Police 'not far away'
The car hit a lamppost, three walls and another car parked on a driveway.
The IPCC said the officers "could not have been that far away", but a spokeswoman said the investigation would find out exactly what happened.
"Issues such as whether or not a police car was following the car, how close it was and whether or not its blue lights were on, for instance, will all be subjects of the inquiry.
"At this stage it is far too early to say what the exact involvement of the police and the vehicle was," she said.