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A man in the US state of Arizona is suspected of killing nine people in three weeks, Phoenix police have said.

Cleophus Cooksey Jr, 35, was arrested on 17 December after he allegedly shot and killed his mother and stepfather.

After Cooksey was in custody, police said they discovered his gun had been used in seven other homicides using new weapons testing technology.

Police say more victims may still be discovered as the investigation continues.

Authorities said they have yet to confirm a motive, or the relationship between the suspect and the victims.

During a press conference on Thursday police described the timeline of the attacks.

On 27 November, Cooksey allegedly shot and killed Andrew Remillard, 27, and Parker Smith, 21.

Days later on 2 December, Salim Richards, 35, was found critically injured at an apartment complex. He died at the scene.

Jesus Bonifacio Real, 25, was found shot to death on 11 December inside a Phoenix residence.

LaTorrie Beckford, 29, an aspiring ambulance worker, was shot to death on 13 December.

On 15 December, Kris Cameron, 21, was found with multiple gunshot wounds in a field near where Mr Beckford had been killed almost exactly 48 hours earlier. He died a day later.

Maria Villaneva, a 43-year-old mother, was shot on 16 December after being sexually assaulted, police say.

On 17 December, police responded to a call of shots fired at an apartment complex in central Phoenix.

An officer noticed what appeared to be blood on a porch and confronted Cooksey inside the apartment, where he was placed under arrest.

Rene Cooksey, 56, and Edward Nunn, 54 - the suspect's mother and stepfather - were found dead on the living room floor.

Officers credit the city's new membership in the Justice Department-led National Crime Gun Intelligence Center Initiative, which makes it easier to track links among gun shells, projectiles and other ballistics.

Arizona Department of Corrections records show Cooksey previously served 16 years in prison for manslaughter and armed robbery convictions.

He was released in July 2017, and neighbours say he had since been living with his mother.