By Lawrence Hurley

WASHINGTON (Reuters) - The U.S. Supreme Court on Monday expanded its ban on mandatory sentences of life in prison without parole for inmates convicted of murders committed before age 18, saying even those imprisoned years ago should have an opportunity to argue for their release.

The court, in a 6-3 ruling, sided with Louisiana inmate Henry Montgomery, who was convicted in the 1963 fatal shooting of a sheriff's deputy at age 17 and has spent more than a half century behind bars with an automatic sentence of life without possibility of parole.

The court in 2012 had ruled that mandatory life sentences without parole in homicide cases involving juvenile killers violated the U.S. Constitution's ban on cruel and unusual punishment. The justices, in their ruling on Monday, said that decision must be applied retroactively to inmates convicted before that ruling was issued.

That means Montgomery and more than 1,000 people serving similar sentences around the United States could be resentenced or given the chance to apply for parole. It does not, however, guarantee their release.

Louisiana, Michigan and Pennsylvania are among the states likely to be most affected.

"For these people, it will be the first time a judge will be able to take into account the qualities that made them, as children, less culpable than adults who commit the same crimes," said Katherine Mattes, director of the criminal litigation clinic at Tulane University Law School in New Orleans.

Montgomery, who is black, was convicted of killing a white sheriff's deputy in East Baton Rouge at a time when racial tensions in the region were boiling over.

The ruling, with conservative Chief Justice John Roberts and Justice Anthony Kennedy joining the court's four liberals, was the court's latest to protect juvenile offenders from the harshest sentences. In 2005, the Supreme Court banned capital punishment for those under 18 at the time of their crimes.

Monday's ruling, authored by Kennedy, left open the possibility of juveniles being sentenced to life without parole in certain cases. Kennedy said Montgomery himself showed "his evolution from a troubled, misguided youth to a model member of the prison community."

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Kennedy emphasized that "children who commit even heinous crimes are capable of change."

Justice Antonin Scalia dissented, along with fellow conservatives Clarence Thomas and Samuel Alito. Scalia called the ruling "a devious way of eliminating life without parole for juvenile offenders."





(Reporting by Lawrence Hurley; Editing by Will Dunham)