That year, Macumber was convicted of two counts of first-degree murder and sentenced to two concurrent life terms. After successfully appealing his convictions, Macumber was retried in 1977 and again found guilty and sentenced to two life terms.
Despite having spent the last 37 years in prison for a crime he said he didn't commit, Macumber said he harbors no resentment.
"I don't know who I have to forgive," he said. "The jury did the only thing they could have done... I have a lot of questions, unfortunately the people who can give me the answer are all dead."
Ron Kempfert was overjoyed when his father walked out of prison.
"We just hugged each other," he said. "There was nothing to be said. It's what we have been waiting for for so long."
Kempfert, now 44, told "Nightline" in 2010 he had no doubt of his father's conviction until the Arizona Justice Project, an organization that works to free prisoners they decide could be innocent, approached him in 2003 and told him they believed his mother framed his father for the murders.
After recovering from the initial shock, Kempfert said he started to untangle what his mother had told him over the years, and slowly the possibility that his father was innocent began to make sense to him. He told "Nightline" he eventually came to believe that his mother had a powerful motive: Around the time she turned her husband into police, Carol Kempfert and Macumber's marriage was falling apart. At the time, she was working in the sheriff's office, where she had access to evidence from the cold case murder, and she had recently taken classes in lifting fingerprints.
Today, Kempfert maintains that she did not frame her husband.
"I passed four polygraphs and I'll be happy to take another, but I did not tamper with any evidence," she said.
But adding fuel to Ron's belief that his father was innocent, a man named Ernsesto Valenzuela allegedly confessed to three different people that he had committed the murders -- evidence the jury at Macumber's trial never was able to hear.
After Valenzuela died in prison in 1973, his former defense attorney, Tom O'Toole, came forward with his client's confession but the judge ruled it unreliable hearsay, reported The New York Times. O'Toole said attorney-client privilege kept him from presenting Valenzuela's confession until after his death.
"I believe [Valenzuela] told me about committing those murders because he got pleasure in committing those kinds of crimes and he relished it," O'Toole said. "He analogized shooting one of those people to it being like shooting a rabbit... he was thriving on it, he loved it."
Over the course of his time in prison, Macumber and his attorneys petitioned the Arizona Board of Executive Clemency unsuccessfully three times. But when they petitioned a fourth time in 2009, the board unanimously recommended his sentence be commuted, a rare move, saying, "An injustice has been done in Mr. Macumber's case" and that his wife had "motive, means and opportunity to falsely pin the murders on Mr. Macumber."
Arizona Gov. Jan Brewer denied the recommendation for clemency.
In 2011, Macumber petitioned the Maricopa County Superior Court for post-conviction relief and was granted an evidentiary hearing. On Tuesday, prosecutors said in court they were unable to retry the case for a third time without the necessary evidence.
Regarding Carol Kempfert, her son Ron said, "I have not spoken to my mother in a long time, and I don't see that in the foreseeable future."
And when asked if she had any relationship with Ron, Kempfert said, "As far as I'm concerned, he's a Macumber, we're Kempferts."
Macumber and Carol Kempfert have two other sons, Steve and Scott. Steve died this June while Macumber was still in prison, and he said he hadn't spoken to Scott in 38 years.
His next plan, Macumber said, is to "spend a couple days going fishing," and he pledged to help the Arizona Justice Project, whom he called his "second family," reviewing cases of other elderly inmates.
"The world has passed me by in four decades," he said. "I am not interested in totally catching up, but I will catch up in the degree that I have to. I am computer literate, I am going to get on the internet and go to work."