The woman whose body was found inside a car on the Mitchell Freeway was a mother-of-three from Scarborough.
Police say the 35-year-old woman was Rebecca Jane Gascoigne and said they were searching for an "extremely dangerous" man in connection to the case. He was believed to have a firearm.
The man wanted for questioning over the incident is Shawn Adam Newton, 42.
Detective Senior Sergeant Dean Bailey said Mr Newton, from Lockridge, should not be approached.
"He is considered extremely dangerous and the public are warned not to approach him," Detective Senior Sergeant Bailey said.
"His whereabouts for us at the moment are unknown."
Police found Ms Gascoigne's body in a Mitsubishi Lancer being driven on the Cedric Street on-ramp at around 12.35am on Saturday morning.
The driver of the Mitsubishi Lancer, a 49-year-old man, is still helping police with their inquiries.
According to an ABC news report, the driver of another car had stopped in the area after being involved in a crash.
That driver reportedly told a news camera operator how he was approached by a man, who informed him he had a body in his car.
Detective Senior Sergeant Bailey confirmed the 49-year-old man had approached police during a separate incident on the freeway.
"The gentleman who is assisting police approached officers at another incident...and identified that the body was in his vehicle," he said.
Camera footage taken at the scene shows a man lying down on the side of the freeway being guarded by a police officer.
The freeway was entirely closed in the early hours between Karrinyup Road and Hutton Street as police established a crime scene, causing chaos for drivers as they sought alternative routes.
Two lanes remained closed as investigating officers cordoned off part of the road, setting up a forensics tent, leaving one lane open and traffic flowing past slowly.
A number of hours later, Main Roads WA confirmed just prior to 2pm on Saturday the area had been cleared and traffic flow had resumed across all lanes.
A post mortem on Ms Gascoigne's body is yet to be conducted.
Detective Senior Sergeant Bailey said Mr Newton and Ms Gascoigne were known to each other.
Police searched two houses in Scarborough on Saturday in connection with the investigation. One of the homes was on Abbett Street, the other on Wilton Place.
Mr Newton is described as light skinned, approximately 186cm tall, with a medium build, very short dark hair and brown eyes.
Members of the public should not to approach him and should call police immediately on 131 444.