Forty-eight Colorado inmates serving life sentences for murders they committed as teenagers will get a new chance at freedom under a U.S. Supreme Court ruling announced Monday.

The court’s decision extends the reach of a 2012 opinion that found it is unconstitutional to sentence juveniles convicted of murder to automatic life terms without the possibility of parole. Heated arguments about whether that ruling applied to those convicted before 2012 circled in the lower courts of seven states — including Colorado — for nearly four years.

But the justices voted 6-3 to settle the debate in their most recent decision, effectively ensuring that more than 1,000 inmates across the country must now be considered for parole or reduced prison time.

In Colorado, the high court’s decision cleared the way for 48 inmates sentenced from 1991 to 2006 to have their sentences reviewed, said Elise Logemann, executive director of the Colorado Juvenile Defender Center.

“We’re very excited that people who were convicted as juveniles will have this opportunity to have these sentences reviewed,” she said. “It will give people who were told as children that they will die in prison a chance to show that they have changed and matured and are ready to re-enter society.”

Monday’s decision stems from the case of Henry Montgomery, who was playing hooky from high school when he encountered and killed a deputy on truant patrol in Baton Rouge, La., in 1963. Montgomery was 17 when he killed the deputy in what he called a “cold panic.”

He has spent more than 50 years in prison. Montgomery works in the prison’s silkscreen department and is a boxing coach.

Justice Anthony Kennedy, who wrote the opinion for the majority, said reconsidering the life sentences does not undermine the convictions themselves. Offering parole hearings to the inmates does not guarantee that they will be released.

“Those prisoners who have shown an inability to reform will continue to serve life sentences,” Kennedy wrote.

But, Kennedy said, inmates such as Montgomery “must be given the opportunity to show their crime did not reflect irreparable corruption; and, if it did not, their hope for some years of life outside prison walls must be restored.”

Under a Colorado law enacted in 1991, anyone convicted of a Class 1 felony — including juveniles — automatically was sentenced to life in prison without the possibility of parole. Class 1 felonies include first-degree murder.

Six years before the U.S. Supreme Court found automatic life sentences for juveniles to be unconstitutional, Colorado legislators passed a law eliminating life sentences for juveniles. Under that law, juveniles convicted of murder must be considered for parole after serving 40 years in prison.

But a clause that would have made the law retroactive was removed after former Gov. Bill Owens raised concerns for families of past victims.

After the justices voted 5-4 in 2012 to strike down automatic life sentences for juveniles, Colorado was one of seven states that refused to apply the Supreme Court’s ruling retroactively. Alabama, Louisiana, Michigan, Minnesota, Montana and Pennsylvania are the other states, according to public interest law firms that advocate on behalf of inmates.

In June 2014, the Colorado Supreme Court heard arguments in four cases involving juvenile defendants. Attorneys for the men argued their clients’ sentences of life without parole were unconstitutional under the 2012 decision from the U.S. Supreme Court.

Attorneys and state justices acknowledged there had been little direction from state lawmakers on how to address those cases.

One year after hearing the arguments, the state’s high court ruled that two of the four sentences should be reviewed because the 2012 ruling applied to their cases.

Neither decision by the U.S. Supreme Court expressly prohibits a judge from sentencing a juvenile to a lifetime in prison. The court has ruled that those sentences should be rare and reserved only for the most heinous crimes.

In his dissent, Justice Antonin Scalia called the ruling “a devious way of eliminating life without parole for juvenile offenders.” Justices Samuel Alito and Clarence Thomas joined Scalia’s dissent.

Logemann, of the Colorado Juvenile Defender Center, said it could take months or years for all 48 Colorado cases to go under review.

“Hopefully it’s fast,” she said.

Jordan Steffen: 303-954-1794, jsteffen@denverpost.com or @jsteffendp

The Associated Press contributed to this report.