A 71-year-old inmate whose case led to a landmark US Supreme Court decision on juvenile-offender sentences was denied parole on Monday, more than half a century after he killed a sheriff’s deputy at the age of 17.

A three-member panel from the Louisiana state parole board voted two to one to keep Henry Montgomery imprisoned. The hearing was his first chance at freedom since his conviction decades ago.

Montgomery now must wait another two years before he can request another parole hearing. A vote to free him would have had to be unanimous.

The Supreme Court’s January 2016 decision in Montgomery’s case opened the door for roughly 2,000 other juvenile offenders to argue for their release after receiving mandatory life-without-parole sentences.

Montgomery has served 54 years in prison for fatally shooting East Baton Rouge Parish sheriff’s deputy Charles Hurt in 1963, less than two weeks after Montgomery’s 17th birthday. Last June, a state judge who re-sentenced Montgomery to life with the possibility of parole called him a “model prisoner” who appears to be rehabilitated.

Montgomery’s lawyers said he has striven to be a positive role model for other prisoners, serving as a coach and trainer for a boxing team he helped form at Louisiana State Penitentiary at Angola.

But the two parole board members who voted against Montgomery questioned why he had not accessed more programs and services that could have benefited him in prison. One of the panellists, Kenneth Loftin, also said he was disappointed in some of Montgomery’s statements during the hearing but did not specify which ones.

Montgomery told the board he has asked for forgiveness from the deputy’s family and from God.

“I’m sorry for all the pain and misery that I’ve caused everybody that’s involved in this case,” he said.

The board also heard from two daughters and a grandson of Mr Hurt, all of whom opposed Montgomery’s release. Mr Hurt was married and had three children.

Linda Woods, his youngest daughter, said she and her sister met with Montgomery in prison last year and forgave him for what he did.

“But I do believe that justice has been done and he needs to stay there,” Ms Woods added.

Montgomery faced the board by video conference from the prison. His lawyer, Keith Nordyke, said Montgomery was “stoic” but very disappointed by the board’s decision.

Mr Nordyke said Montgomery completed every course required by law to be eligible for parole. However, during his first three decades behind bars, prisoners at Angola were prohibited from taking any courses.

“He just did what people on the outside do every day, which is go to work and not get in trouble,” Mr Nordyke said.

Montgomery initially was sentenced to death after a jury convicted him. After the Louisiana Supreme Court ruled he did not get a fair trial and threw out his murder conviction in 1966, he was retried, found “guilty without capital punishment” and automatically sentenced to life without parole.

In 2012, however, the US Supreme Court ruled that sentencing juvenile homicide offenders to life without the possibility of parole is unconstitutional “cruel and unusual” punishment. In January 2016, the justices made their decision retroactive, deciding in Montgomery v. Louisiana to extend its ban on such sentences to people already in prison.

During Monday’s hearing, Mr Loftin asked Montgomery if he remembered the shooting.

“Everything happened so fast, and I wasn’t intending to kill nobody,” Montgomery replied.