The U.S. Supreme Court could save the lives of up to 20 death row inmates via an Arizona case it took up last week.

Their fates depend on the justices' final answer to a question that other courts have split over: do current laws apply when reviewing old capital sentences?

James McKinney, the petitioner in the case, was sentenced to death by a judge in September 1993 for committing two gruesome killings. But according to a 2002 Supreme Court ruling called Ring v. Arizona, juries, not judges, must be the decision-making bodies weighing the facts for and against death sentences.

After the Ninth Circuit ruled four years ago that Arizona had erred in his case because its sentencing procedures prevented mitigating factors like his post traumatic stress disorder, or PTSD, from being considered, McKinney thought Ring guaranteed that he'd be resentenced by a jury.

But the Arizona Supreme Court disagreed, noting that Ring was decided years after McKinney's sentence was final. Instead of the jury resentencing hearing he sought — complete with a more developed record about his horrific childhood — the state high court simply reevaluated the old trial court evidence and affirmed the original sentence last September.

McKinney appealed to the Supreme Court, arguing that current laws should have applied and that a jury, not the state's highest court, should have decided whether his PTSD merited a lesser sentence.

In a March amicus brief supporting him, the Arizona Capital Representation Project argued that the state high court's decision to independently review McKinney's sentence means "life or death decisions [are] founded on outdated unreliable science."

"They're looking at a cold record of evidence that was proffered over 20 years ago," said Amy Armstrong, the organization's director. "When you think about the scientific and popular understanding of things like trauma and PTSD, now versus 20 years ago ... it's very different."

She added that a favorable ruling on the case could force Arizona to resentence up to 20 other death row inmates who were similarly sentenced without a jury having considered certain kinds of mitigating evidence, like PTSD.

"They don't want to fix all of this," Armstrong told Law360, explaining the state's unwillingness to grant McKinney a jury resentencing. "I'm sure they're looking at it like, 'this is a lot of cases that are going to be undone.'"

Ben Cohen of the Promise of Justice Initiative also filed an amicus brief in the case, pointedly noting that McKinney is "one of the few defendants on death row in America sentenced to death by a judge, not a jury."

He added that science has shown PTSD diminishes moral culpability because it actually changes someone's brain composition. A jury should hear the evidence of McKinney's childhood, he said, because "we now know so much more about the impact of trauma than we did ... when he was tried and sentenced to death."

'If our justice system doesn't take that into account," Cohen said, "it reveals our inhumanity, our indecency, not his."

In McKinney's case, his father was an alcoholic and his mother allegedly locked him and his siblings in closets. After his father took McKinney and moved in with another woman, McKinney's aunt testified, "the kids ... never had clean clothes that I ever saw them in" and were forced to share a bedroom with dogs, cats, snakes, a goat and a monkey — a menagerie that "regularly defecated and urinated in the bedroom."

The woman also allegedly beat him and his siblings. McKinney ultimately dropped out of school in seventh grade after years of harassment from classmates and repeatedly tried to run away from home.

By age 23, he'd resorted to burglaries, including two that turned deadly in March 1991. In the first, McKinney and his half-brother Charles Hedlund brutally beat and shot 40-year-old Christine Mertens in her home, making off with about $120. Two weeks later the pair killed 65-year-old James McClain in his sleep and stole his watch, some guns and his car.

The killings and his ultimate conviction occurred during a time when the state applied a "causal nexus" rule at sentencing, prohibiting consideration of mitigating evidence if it was unconnected to the crime in question.

As a result, the sentencing judge did not consider a psychologist's testimony that McKinney suffered from a "horrific childhood" as mitigating evidence worth leniency.

Arizona's Supreme Court affirmed his death sentence on appeal, leaving McKinney no option but to file a habeas corpus petition in federal court. That bid ultimately reached the Ninth Circuit, which ruled in 2015 that Arizona's causal nexus rule had violated defendants' constitutional rights to have mitigating factors considered by their sentencers.

"The Arizona Supreme Court's refusal, as a matter of law, to give weight to [McKinney's] PTSD, requires resentencing," the Ninth Circuit wrote.

But when the case was remanded, the Arizona Supreme Court decided to independently review McKinney's sentence. Though he'd asked for a jury, citing the 2002 Ring ruling, the state high court said his case was final before Ring was issued.

McKinney's Supreme Court case looms large for his half-brother Hedlund, who was also sentenced to death for his role in the killings despite evidence of PTSD.

Though the Ninth Circuit overturned Hedlund's sentence months after overturning McKinney's, the Arizona Supreme Court independently reviewed and affirmed it, saying Ring's jury requirement didn't apply to his case either.

Therein lies the rub: McKinney's Supreme Court bid notes the Florida and Washington supreme courts as well as the First, Second and Fourth circuits have held that current law applies to resentencing or sentence corrections.

Arizona's position, however, is shared by the Seventh Circuit, which has also expressed the view that "current law" does not always apply to post-remand proceedings.

According to the state's April brief urging the Supreme Court to pass on the case, the fact that its high court conducted an independent review instead of a jury review "should have surprised no one" because state statutes require "the supreme court shall independently review findings of aggravation and mitigation and the propriety of the death sentence."

Agreeing with McKinney's claim that a case is not final as long as a sentence remains open to correction, Arizona argued, would mean "there is no such thing as 'finality' in capital cases."

"By any principled measure, [McKinney's] convictions and sentences for killing two blameless and essentially incapacitated victims became final years before this court decided Ring v. Arizona," the state said.

Katie Conner, a spokesperson for the Arizona attorney general's office, told Law360 that "there is no need for the U.S. Supreme Court to get involved at this stage."

"That being said, we feel confident in our arguments and believe we will ultimately prevail at the federal level as well," she added.

Neal Katyal, a Hogan Lovells attorney representing McKinney, said he looks forward to arguments in the fall.

"We are glad that the Supreme Court agreed to hear this important case," Katyal said.

--Editing by Brian Baresch.

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Clarification: this article has been updated to clarify the Ninth Circuit's ruling.