NEWARK — Moments after watching a 15-year-old boy receive a 40-year prison sentence for fatally shooting three teenagers on Christmas night last year, the victims' families today said the sentence wasn't long enough and criticized prosecutors' decision to try him as a juvenile, rather than in adult court.

"I ain't satisfied with the justice," said James Hailey, the father of Zainee Hailey, who was gunned down while taking out the trash on Schley Street in Newark.

"He should have got life," Hailey said.

The boy received the sentence after pleading guilty on Sept. 8 to a juvenile petition charging him with three counts of murder in the shooting deaths of Zainee Hailey, Kasson Morman and Abdul Frazier.

Hailey, 13, was shot as she and her younger brother carried trash bags to the street from her family's third-floor apartment. Hailey, and Morman, 15, died shortly after the attack, and Frazier, 14, died from his injuries a few weeks later.

The shooter opened fire, because he felt "disrespected" after one of the boys apparently hit on his girlfriend, law enforcement sources have said.

As part of the plea agreement, the juvenile also pleaded guilty to a weapons offense related to the Dec. 21, 2013 fatal shooting of Reginald Terry, according to Essex County Assistant Prosecutor Purva Deshpande, who handled the case.

The cases against two other individuals charged in Terry's murder are pending in adult court, Deshpande said.

The juvenile's sentencing hearing was closed to the media.

Although the boy received a 40-year prison sentence, Deshpande said there's no minimum that the juvenile would have to serve before becoming eligible for parole. "It's indeterminate, so there's no way of knowing exactly how much he's going to serve," she said.

Laura Cohen, a professor at Rutgers School of Law in Newark, said the juvenile would almost certainly serve at least a third of his sentence, and it's highly likely he will serve much longer than that.

As compared to the adult correctional system, the juvenile system includes a greater emphasis on rehabilitation, Cohen said. That rehabilitative approach includes educational programs and mental health and substance abuse treatment, she said.

James Hailey looks over the casket as a funeral was held for his daughter, Zainee Hailey, at the New Zion Baptist Church in Elizabeth on Tuesday, December, 31, 2013. (Ed Murray/NJ Advance Media for NJ.com)

"The juvenile system...is founded on the principle that young people are still changing," said Cohen, adding that the system is meant to hold juveniles "accountable for their wrongdoing, but it';s also intended to address whatever the problems or issues were that brought children into the court in the first place and hopefully prevent them from making the same mistakes in the future."

The juvenile's name in the triple murder on Schley Street has not been released, because he was not waived up to adult court. His attorney, Paula Getty, declined to comment after today's sentencing.

Prosecutors had begun the process of waiving the juvenile's case up to adult court, but ultimately decided to accept the guilty plea in juvenile court after weighing the likelihood of conviction in adult court against the boy's willingness to accept the plea deal, Acting Essex County Prosecutor Carolyn Murray said in a Sept. 10 interview.

"It's a professional weighing of factors in every case," Murray said. "There are factors in a juvenile case that aren't available to the public...all I can really say is those factors are known to us and weighed by us and this was, in my judgment, the best outcome for this case."

But some members of the victims' families today blasted that decision, saying the juvenile should have been tried in adult court.

"He was a juvenile when he did it, but he had adult intentions,"; said Steven Williams, the father of Abdul Frazier. "Kids don't think about killing kids...Adults think about killing people."

However, Williams later said in a series of emails that, given the circumstances, what happened was the "best deal." If the juvenile's case had been moved to adult court, "There was a greater chance of him walking without any type of punishment," Williams said.

"At this point I just hope this boy can do his punishment he is given and turn his life around," Williams added.

Shaheedah Frazier, Abdul's mother, said after today's hearing that the juvenile deserved a longer prison sentence. "It's not enough," she said. "It's just not enough."

After the sentencing, Deshpande said "that 40 years...is not meant to represent the loss of any child in this case."

"It's basically looking at all the circumstances that we had here, the state's proofs, in coming to this," said Deshpande, adding that "looking at all the factors in this case, looking at all the proofs, we felt that this was the most appropriate sentence, given everything."

Bill Wichert may be reached at bwichert@njadvancemedia.com. Follow him on Twitter @BillWichertNJ. Find NJ.com on Facebook.