More than a thousand inmates in the nation's prisons who were sentenced as juveniles to life without the possibility of parole can now challenge those punishments, the U.S. Supreme Court ruled Monday.
The decision extended a 2012 ruling, which invalidated future life-without- parole sentence for juvenile murderers, to all such offenders who were given life sentences in the past.
Monday's case involved Henry Montgomery, a Louisiana man who at age 17 killed a deputy sheriff in East Baton Rouge. The court in his trial was barred by law from considering arguments that his age should matter, "including evidence that as a scared youth, Mr. Montgomery shot in panic as the officer confronted him playing hooky," his lawyers said.
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As a general matter, rulings do not apply retroactively. The courts have long held that society has an interest in the finality of convictions.
But by a vote of 6-3, the Supreme Court said its 2012 ruling fit in a special category of exceptions applying to decisions that ban certain forms of punishment for a class of offenders because of their status.
Monday's opinion was written by Justice Anthony Kennedy and joined by Chief Justice John Roberts and the court's four liberals, Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor, and Elena Kagan.
A total of 2,341 people are now serving mandatory sentences of life without parole for juvenile offenses. Roughly 1,000 of them would be affected by a decision in this case, according to a study by The Phillips Black Project, a non-profit law group that represents prisoners facing severe sentences.
The remainder, the study concluded, were imprisoned in states that have already applied the ruling retroactively.
Monday's decision said the states do not have to hold new sentencing hearings if they allow juvenile homicide offenders the opportunity to be released on parole.
Such a step "ensures that juveniles whose crimes reflected only transient immaturity -- and who have since matured -- will not be forced to serve a disproportionate sentence" in violation of the Constitution's ban on cruel and unusual punishment.
By contrast, the court said, "Those prisoners who have shown an inability to reform will continue to serve life sentences."
This article first appeared on NBCNews.com.