The woman whose body was found inside a car on the Mitchell Freeway was a mother-of-three from the Scarborough area.

Police revealed today the 35-year-old woman's name was Rebecca Jane Gascoigne, and said they were searching for a "very dangerous" man believed to have a firearm.

The man wanted for questioning over the incident is Shawn Adam Newton, 42.

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.

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.

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.

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.