Caoimhin Cassidy had not been seriously injured in the crash, police have said.
And they say that if “they had a conscience they would come forward”
Caoimhin Cassidy died in Derry on Saturday, June 1 after the vehicle crashed into a lamp post.
His great-uncle, Charles Tierney, said those who attended the funeral have not helped with the police investigation.
“They haven’t come forward, they aren’t friends,” Mr Tierney said.
“If they had a conscience, they will come forward.
“We don’t want young people to go to prison. We just want to know what happened.”
A post mortem examination revealed Mr Cassidy had not been seriously injured in the crash and died after the car caught fire on Fairview Road.
The police said the car, a read Mazda 6, was stolen from a house in Oakfield Crescent earlier in the day.
Mr Tierney said people who have been posting on social media the names of people who they believe were involved are “disrupting the investigation”.
“If people think they know who is involved, just come forward.”
Caoimhin Cassidy’s body was found inside the Mazda 6 car in the Galliagh area of Derry on June 1
Detective Inspector Michael Winters said police believed Mr Cassidy was not travelling alone in the vehicle.
“We’ve received a report of two males running away from where the vehicle came to a final halt,” he said.
“We’ve also been made aware of sightings of a male, possibly injured, walking on the Buncrana Road, past the Skeoge Link Road, towards the border a short time later.
“If you can cast your mind back and remember anything about Caoimhin’s movements, or the red Mazda 6 car, please get in touch,” added DI Winters.
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