The US Supreme Court said Monday that its landmark 2012 ruling banning the sentencing of minors to life in prison without parole can be applied retroactively.
The decision offers hope of early release for more than 2,000 prisoners who were sentenced to life without parole before 2012 for crimes committed when they were minors.
Voting six to three, the nine justices ruled in favor of Henry Montgomery, who had asked to be resentenced in keeping with the US Constitution's Eighth Amendment ban on "cruel and unusual punishment."
When he was 17, Montgomery shot and killed a deputy sheriff in Louisiana. He has been locked up since 1963.
Monday's decision will have major repercussions on the US prison system.
Human Rights Watch estimates there are at least 2,225 people serving life sentences without parole for crimes committed before they were 18.
The Supreme Court ruled on June 25, 2012 that sentencing an adolescent to life without parole was a violation of the constitution, requiring that underage criminals be treated differently than adults.
But the historic ruling initially benefitted only minors convicted of murders committed after that ruling.
The latest decision is part of a larger debate over harsh sentencing guidelines and their impact on the overcrowded US prison system.