An 18-year-old woman has died after attending a medical centre in the southern Perth suburb of Spearwood with serious injuries.
About 4:30pm on Tuesday she went to the doctor's office, from where she was taken to Fiona Stanley Hospital, where she died.
Homicide squad officers have been at a home in Galian Way.
"A 17-year-old male is currently assisting investigators with their inquiries," a WA police statement said.
Neighbours said they had often seen the pair together and they used to wear school uniforms.
One resident, who asked not to be named, said he had been at the Spearwood Chemist on Tuesday night when he saw a large police presence there.
When he returned home he thought perhaps his neighbours were having a party.
The resident said they only realised on Wednesday morning that the cars he had seen were undercover police vehicles.
"I'm just shocked and thinking if I had heard something maybe something could have been done," they said.
"It's so close you know, just two doors down."
Anyone with information about the incident is being asked to contact Crime Stoppers on 1800 333 000.
