Milke was a 25-year-old insurance company clerk when her son was killed. She has maintained her innocence, saying she had nothing to do with the slaying.
The two men convicted in the case both remain on death row. Neither Roger Scott nor former Milke roommate James Styers testified at Milke's trial. Scott confessed during a police interrogation and led detectives to the boy's body.
Maricopa County prosecutors are still seeking the death penalty against Milke at her retrial, tentatively set for Sept. 30, and her alleged confession is at the heart of the case against her.
Police detective Armando Saldate Jr. testified that she confessed to him in a closed interrogation room. But the confession was not recorded.
At trial, Milke denied that she had confessed, but the jury believed the detective.
Doubts about Saldate's honesty arose during Milke's appeals. The 9th Circuit concluded in March that prosecutors' failure to turn over evidence related to Saldate's credibility deprived Milke's attorneys of the chance to question his truthfulness before jurors.
"No civilized system of justice should have to depend on such flimsy evidence, quite possibly tainted by dishonesty or overzealousness, to decide whether to take someone's life or liberty," Chief Judge Alex Kozinski wrote for the court.
The court noted four cases in which judges threw out confessions or indictments because Saldate lied under oath and four instances in which cases were tossed out or confessions excluded because Saldate violated the suspect's constitutional rights.
He was also suspended for accepting sexual favors from a female motorist he stopped and then lying about the encounter, the court said.
Deputy County Attorney Vince Imbordino argued last week during a bond hearing that the purported confession is still admissible, but Judge Rosa Mroz of Maricopa County Superior Court said the undisclosed material concerning Saldate "casts serious doubt" on its validity.
Mroz, who also set Milke's bond, scheduled a Sept. 23 hearing on the defense's request to prohibit the prosecution from using the confession during the retrial.