'Extremely dangerous' man wanted after Perth mum found dead in car

An "extremely dangerous" man possibly armed with a gun is wanted by police, after a Perth mother was found dead in a car on a freeway.

Rebecca Jane Gascoigne's body was found in a Mitsubishi Lancer on the Mitchell Freeway near the Cedric Street off-ramp about 12.35am Saturday.

WA Police yesterday released images of the 35-year-old Scarborough mum-of-three and of a man they wish to speak to over her death, Shawn Adam Newton.

"He is considered extremely dangerous, the public are warned not to approach him as he is in possession of a firearm," Homicide Squad Detective Senior Sergeant Dean Bailey said .

Mr Newton and Ms Gascoigne knew each other, but Det. Snr Sgt Bailey would not say how.

Police currently have a 49-year-old man in custody, helping them with their investigation.

"A gentlemen who's assisting police approached officers at another incident on the Cedric Street ramp and identified that the body was in his vehicle," Det Snr Sgt Bailey said.

Mr Newton, 42 of Lockridge, is described as fair skinned, about 186cm tall, with a medium build, shaved dark hair and brown eyes.

Anyone with information is asked to call police on 131 444 or Crime Stoppers on 1800 333 000.