A 22-year-old New Orleans man pleaded guilty to manslaughter Tuesday and accepted a 35-year prison sentence for a retaliatory killing committed when he was 16.

Arthur Grandpre was arrested six years ago in the Feb. 4, 2010, murder of David Neiswonger. Authorities described it as a hit job on a witness to an earlier killing that took place in the same house in the 2300 block of D’Abadie Street in the 7th Ward.

They accused Grandpre of accepting the hit job and firing the fatal shot in a slaying for which three others also were charged with murder.

One of those men, John Cunningham, pleaded guilty in 2012 to both the earlier slaying and the killing of Neiswonger. Cunningham, who was accused of ordering the hit from jail, accepted a 70-year prison sentence.

Two others, Gerald Williams and Phil Dominick, still await trial.

Grandpre’s attorneys, Michael Kennedy and Miles Swanson, said they were “extremely pleased” with the outcome for their client, while arguing that he should have been tried as a juvenile.

Grandpre, who was arrested within weeks of Neiswonger’s killing, was among the longest-held pre-trial detainees in Orleans Parish before his guilty plea.

“We are talking about someone who was arrested as a child, who has spent every day of his adult life in jail,” Kennedy said in a statement. “Today he made his one adult decision ever. And when he has served his time and is released from the Department of Corrections, he still will essentially be that 16-year-old boy who has never had a job or paid bills or had to decide what to eat or when to go to bed or anything else.”

Kennedy suggested that Grandpre’s prosecution as an adult lends fodder to a campaign that juvenile justice reform advocates are waging to raise the age for which juveniles may be tried as adults in Louisiana.

Anyone 17 or older is an adult in the eyes of the law in Louisiana, one of nine states that don’t set the bar for adult criminal suspects at 18. Anyone younger normally is tried in Juvenile Court, but teens who are 15 or 16 can be transferred to adult court at the district attorney’s discretion if they are charged with specific serious offenses.

In Orleans Parish, District Attorney Leon Cannizzaro’s office has been far more prone than his predecessors to transfer such accused juveniles to adult court.

In a statement, Cannizzaro described Grandpre as a prime example for why.

“This was truly a horrific crime. The actions of this defendant were deliberate and constituted a clear and present danger to the public safety of this community,” Cannizzaro said. “Some critics have accused my office of transferring too many cases to Criminal District Court for prosecution but have declined to identify those cases that they believe should have remained in Juvenile Court. I must ask my critics: Is this one of those cases?”

Cannizzaro noted that, had Grandpre been convicted of murder and sentenced to a life prison term, a judge could have seen fit to make him eligible for parole in about the same 35 years as the term he accepted, under a 2012 U.S. Supreme Court decision barring mandatory life terms for juveniles.

A revolving door of defense attorneys — Grandpre had five of them — was partly to blame for the lengthy delay in a trial.

Follow John Simerman, on Twitter @johnsimerman.