The boy was undergoing surgery at Westmead Hospital on Tuesday morning and remained in a critical condition.
“This is a tragic, tragic incident that has occurred, essentially over nothing; the purchasing of vapes. A young man has been seriously, seriously injured,” Inspector Laird said.
He said police had spoken to three people who had been in the car and were co-operating with inquiries.
But they are urging a man, aged 18, to come forward to Caste Hill police station so that police can “find out how the 17-year-old ended up under the car”.
That 18-year-old man left the vehicle after the incident, while the other three went back to provide assistance to the injured teenager.
“We think the majority of this whole purchase of vapes centred around him,” Inspector Laird said.
He said the teenager had suffered “catastrophic injuries” and might not be able to tell police what happened.
“The boy only turned 17 the other day; it’s a difficult time of the year for anyone to be involved in tragic traumatic injuries,” he said.
“We owe it to him to investigate all lines of inquiry and we’ll be open to whether it’s a motor vehicle collision or something more sinister, but we won’t rule anything out.”
Detectives are also door-knocking around the neighbourhood and requesting anyone with dash cam footage of the incident to come forward.
