An "extremely dangerous" man is wanted over the death of a Perth mum.

An “extremely dangerous” man armed with a gun is wanted over the death of a Perth mum.

Rebecca Jane Gascoigne, 35, was found dead in a Mitsubishi Lancer on the Mitchell Freeway near the Cedric Street off-ramp about 12.35am on Saturday.

WA Police on Sunday released images of the Scarborough mum-of-three and of Shawn Adam Newton, who they want to talk to over Ms Gascoigne’s death.

Police hunt ‘very dangerous’ man as woman found dead in car named https://t.co/vDagcMLEQ5 via @watoday

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“He is considered extremely dangerous, the public are warned not to approach him as he is in possession of a firearm,” the Homicide Squad’s Detective Senior Sergeant Dean Bailey told reporters on Sunday.

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 gentleman 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.

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

Anyone who knows of Newton's whereabouts is asked to call police on 131 444 or Crime Stoppers on 1800 333 000.