A jury has spared Florida school gunman Nikolas Cruz from the death penalty for killing 17 people at a high school in 2018, sending him to prison for the rest of his life in a decision that angered many families of the victims.

“This is insane. Everyone knows right? This is insane,” Chen Wang, cousin of victim Peter Wang, said at a news conference after the jury’s decision was read. “We need justice.”

Cruz, 24, pleaded guilty a year ago to murdering 14 students and three staff members, and wounding 17 others, at Marjory Stoneman Douglas High School on February 14 2018.

The three-month trial to determine whether he should be executed included graphic videos and photos from the massacre and its aftermath, heart-wrenching testimony from victims’ family members and a tour of the still blood-spattered building.

The jury rejected the death penalty after deliberating for about seven hours over two days.

“We are beyond disappointed with the outcome today,” said Lori Alhadeff, whose daughter, Alyssa, was killed.

“This should have been the death penalty, 100%. … I sent my daughter to school and she was shot eight times. I am so beyond disappointed and frustrated with this outcome. I cannot understand. I just don’t understand.”

Under Florida law, a death sentence requires a unanimous vote on at least one count.

The jury found there were aggravating factors to warrant the death penalty for each victim, such as agreeing that the murders were “especially heinous, atrocious, or cruel”.

But one or more jurors also found mitigating factors, such as untreated issues he had as a child. In the end, the jury could not agree that the aggravating factors outweighed the mitigating ones, so Cruz will get life without parole.

Circuit Judge Elizabeth Scherer will formally issue the life sentences on November 1. Relatives, along with the students and teachers Cruz wounded, will be given the opportunity to speak.

Cruz, his hair unkempt, largely sat hunched over and stared at the table as the jury’s decisions were read.

Rumblings grew from the family section — packed with about three dozen parents, spouses and other relatives of the victims — as it became apparent the jury was not going to recommend the death penalty.

Many shook their heads, looked angry or covered their eyes as the judge spent 50 minutes reading the jury’s decision for each victim. Some parents sobbed as they left court.

Jury foreman Benjamin Thomas, in an interview broadcast on local TV station WPLG, indicated that more than one juror voted for life in prison instead of the death penalty.

“We went through all the evidence and some of the jurors just felt that was the appropriate sentence,” Mr Thomas said. “I didn’t vote that way, so I’m not happy with how it worked out, but everyone has the right to decide for themselves.”

He said the jurors reached their decision on Wednesday, then went home to sleep on it before bringing it to the judge on Thursday.

He said he feels bad for the families of victims and that “it hurt” to watch the decisions being read in court. “There’s nothing we could do. It’s the way the law is. And that’s how we voted,” he said.

“This has been really hard on my heart … I’d rather not see anything like this ever again.”

Tony Montalto, father of Gina Montalto, expressed disbelief at the finding. He said the jury recognised that Cruz committed terrible acts, “shooting, some victims more than once on a pass, pressing the barrel of his weapon to my daughter’s chest. That doesn’t outweigh that poor little what’s-his-name had a tough upbringing?”

“Our justice system should have been used to punish this shooter to the fullest extent of the law,” he said.

Mr Montalto, president of Stand With Parkland, which represents most of the families and other relatives, also said the decision would do nothing to deter another person from gunning down multiple people in a school.

Michael Schulman, the stepfather of teacher Scott Beigel, said the decision gives anyone a licence to kill, then claim mental illness as a defence. “This animal deserves to die. He hunted all of these people,” Mr Schulman said. “He planned it for months.”

As he spoke to the media, Mr Schulman held up a laptop with an image of Cruz in the school hallway with a gun. “The last thing my son saw was the gunman aiming at him,” he said.

Florida Governor Ron DeSantis said it “stings” that Cruz did not receive the death penalty.

In a case like this “where you’re massacring those students with premeditation in utter disregard for basic humanity … I just don’t think anything else is appropriate except a capital sentence”, Mr DeSantis said.

Multiple family members called Cruz a “monster” and talked about the years they waited for justice. They thanked prosecutors for their work but in the end, they said, they got another blow.

Just days after the shooting, Cruz’s defense had offered that he plead guilty in exchange for a life sentence. But then-Broward state attorney Mike Satz rejected that.

Broward State Attorney Harold Pryor released a statement on Thursday saying: “We have not shied away from telling all of the horror, all of the loss, all of the devastation, all of the pain, all of the facts, all of the truth. We hope that, while there is no such thing as closure, this will bring some measure of finality and justice to this terrible chapter.”

Cruz, a former student at the school, said he chose Valentine’s Day to make it impossible for Stoneman Douglas students to celebrate the holiday again.

Cruz’s lead attorney Melisa McNeill and her team never questioned the horror he inflicted, but focused on their belief that his birth mother’s heavy drinking during pregnancy left him with foetal alcohol spectrum disorder.

Their experts said his bizarre, troubling and sometimes violent behaviour starting at the age of two was misdiagnosed as attention-deficit hyperactivity disorder, meaning he never got the proper treatment. That left his widowed adoptive mother overwhelmed, they said.