US police say they have evidence linking a 35-year-old man already charged with two killings to seven additional deaths that occurred in a three-week span late last year.

Police officials in Arizona for Phoenix and the suburbs of Glendale and Avondale said on Thursday that Cleophus Cooksey knew some of the victims but investigators are still trying to determine motives for at least some of the killings.

Cooksey was arrested on December 17 for the shooting deaths of his mother and a man and he is jailed on two counts of first-degree murder and one of being a felon in possession of a weapon.

Investigators were able to use evidence from shelling casings to connect at least some of the killings, officials said.

Phoenix mayor Greg Stanton said new technology allows police to get results of those checks within hours instead of weeks.