Cleophus Cooksey Jr, 35, is accused of an array of killings in Arizona in three weeks during the closing months of last year.
Investigators say he shot dead seven men and two women in their homes, suburban apartment complexes, in a parked car or while outside between 27 November and 17 December.
The murder weapon in one of the attacks was the victim’s own gun, they allege.
Officials in Phoenix, Glendale and Avondale say Cooksey, described as an aspiring musician, knew some of the victims.
But detectives are still trying to determine the motives in a few of the attacks.
The victims include Jesus Real, 25, the brother of Cooksey’s ex-girlfriend.
He was asleep in his home in Avondale on 11 December when he was shot twice in the face.
Avondale police say Mr Real’s sister and Cooksey had broken up the night before.
Another victim, Maria Villanueva, was seen arriving at a Glendale block of flats on 15 December and then leaving with an unknown man.
Her partially naked body was found in an alley in Phoenix the next morning and her car was found abandoned in Glendale.
Investigators found a mobile phone and a bloody man’s shirt inside.
DNA on the shirt matched Cooksey’s, according to court documents. The mobile phone was also confirmed to be his.
Cooksey was arrested on 17 December after allegedly shooting dead his mother and stepfather in their home.
He was initially charged with two counts of first-degree murder and one count of being a felon in possession of a weapon but he has now been arrested over the other seven murders.
Sgt Jonathan Howard, a Phoenix police spokesman, said: “It should appal every one of us in the room that he managed to kill nine people period, let alone in such a short period of three weeks.”
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Glendale police Chief Rick St John said: “I’m just proud as heck that he’s off the street.”
Cooksey previously served 16 years in prison for manslaughter and armed robbery and had been free for 18 months.