The ruling marked a surprising turn in a case that made national headlines with the brazen and gruesome nature of the crime. Prosecutors said Debra Jean Milke dressed up her son Christopher in his favorite outfit and told him he was going to see Santa Claus at a mall during the holidays.
Instead, he was taken into the desert by her boyfriend and another man and shot three times in the back of the head as part of what prosecutors said was a plot by Milke and the two other defendants to collect a $50,000 life insurance policy.
Milke would have been the first woman executed in Arizona since the 1930s had her appeals run out. The Arizona Supreme Court had gone so far to issue a death warrant for Milke in 1997, but the execution was delayed because she had yet to exhaust federal appeals.
A three-judge panel of the 9th U.S. Circuit Court of Appeals ruled Thursday that the prosecution failed to disclose information about a history of misconduct by a detective who testified that Milke confessed to plotting her son's murder.
That record included multiple court rulings in other cases that former Detective Armando Saldate Jr. either lied under oath or violated suspects' Miranda rights during interrogations.
Prosecutors are required to provide a defendant's lawyers with material that might support a not-guilty verdict, including material that could undermine the credibility of a prosecution witness.
There was no other witness or recording of the purported confession by Milke, who has proclaimed her innocence.
"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 Justice Alex Kozinski wrote in the decision.
The trial amounted to "a swearing contest" in which the judge and jury ultimately believed the detective over Milke, but they didn't know of his record of dishonesty and misconduct, Kozinski wrote.
The ruling reversed a U.S. District Court judge's ruling and ordered the lower court to require Arizona authorities to turn over all relevant personnel records for the detective.
Once the material is produced and defense lawyers have time to review it, prosecutors will have 30 days to decide whether to retry her. If they don't, she will be released from prison.
Maricopa County prosecutors had yet to read the ruling and had no immediate comment on the decision, spokesman Jerry Cobb said.
Milke defense lawyer Michael Kimerer was in trial and not immediately available for comment.
In 2009, Kimerer said his client maintains her innocence and was a loving mother who still grieves her son's death.
"Our main concern is the fact that I have a client that never confessed and a police detective who said she gave a confession," Kimerer said then. "There was no tape recorder, no witnesses, nothing. Just his word."
Milke, 48, is one of three women on death row in Arizona. All three are imprisoned at the state prison for women in in Goodyear.
The two men convicted in the Milke's case, Roger Scott and former Milke roommate James Styers, are both on death row at a prison in Florence.
Scott confessed during a police interrogation and led detectives to the boy's body.
But neither Scott or Styers would testify against Milke.
Associated Press writer Amanda Lee Myers contributed.