Police are investigating the sexual assault of a teenage boy by three men at Beulah Park in Adelaide's east early on Sunday morning.
The 15-year-old was walking west along Beulah Road when he was attacked by the men near Brand Street about 2:00am.
Police say the men had been travelling in a white van in the opposite direction but turned around and began following the boy.
Two of them then held him down while the other assaulted him.
Detective Superintendent Damian Powell says the boy was staying with relatives at the time and was on his way back when he was assaulted.
He says the boy was able to give a detailed description of the vehicle used by his attackers.
"A white tradesman's-style van. It has tubing on the roof and a ladder that was situated in the centre of the roof," he said.
"It had a black stripe about four centimetres wide that ran horizontally along the full length of the side.
"Understandably, it's a traumatic experience for any victim of a serious sexual assault and it's no different in this case.
"We are certainly concerned that this is a serious sexual assault. The nature and the circumstances do cause police concern."
Superintendent Powell says detectives have carried out doorknocks in the area and examined the crime scene.
"At this stage we're awaiting the results of forensic analysis to identify whether it assists us to identify those responsible."