(CNN) -- Arizona will appeal a judge's decision to overturn the murder conviction and death sentence of Debra Milke, the state's attorney general announced Friday.
A jury convicted Milke of murder, conspiracy to commit murder, child abuse and kidnapping on October 12, 1990, less than a year after her 4-year-old son was found dead. She was sentenced to death a few months later.
But on Thursday, Chief Judge Alex Kozinski of the 9th U.S. Circuit Court of Appeals overturned her conviction after determining she did not receive a fair trial.
His rationale was that prosecutors did not disclose the "history of misconduct" of its key witness, a Phoenix police detective, who had testified that Milke confessed to being involved in a plot to kill her son.
Unbeknownst to the defense or to the jury, previous judges had tossed out four confessions or indictments because Detective Armando Saldate Jr. had lied under oath, among other issues.
But Arizona Attorney General Tom Horne challenged Kozinski's decision, vowing Friday to personally argue on the state's behalf if the U.S. Supreme Court takes the appeal.
"Ms. Milke was found to have arranged the killing of her own son, a four-year-old toddler, because he was too much of a burden and interfering with her life," Horne said in a written statement. "... This is a horrible crime. The Ninth Circuit's decision needs to be reversed, and justice for Christopher needs to be served."
A day after seeing Santa Claus at a mall, young Christopher Milke asked his mother if he could go again. That was the plan, she said, when the boy got into the car with Milke's roommate, James Styers.
Styers picked up a friend, "but instead of heading to the mall, the two men drove the boy out of town to a secluded ravine, where Styers shot Christopher three times in the head," according to Kozinski's summary of the case. Styers was convicted of first-degree murder in the boy's killing and sentenced to death.
During her trial, "no ... witnesses or direct evidence (linked) Milke to the crime" other than Saldate
The detective questioned Milke -- an interrogation that wasn't recorded or witnessed by anyone else -- and later claimed she'd confessed to her role in the murder conspiracy, saying it was a "bad judgment call."
But Milke offered a vastly different view of the interrogation and denied that she had admitted to any role in a murder plot.
"The judge and jury believed Saldate," Kozinski wrote Thursday. "But they didn't know about Saldate's long history of lying under oath and other misconduct."
Kozinski, chiding prosecutors for remaining "unconstitutionally silent" about the detective's background, overturned the conviction and set the stage for Milke's release from prison unless the state quickly presses forward with a retrial.
Horne, the Arizona attorney general, argues that the 49-year-old woman should remain on death row, given his understanding of what happened.
"After dressing him up and telling him he was going to the mall to see Santa Claus, Milke was convicted of sending her young son off to be shot, execution style, in a desert wash," he said.
CNN's Deanna Hackney contributed to this report.