Jacob Ind got the idea of killing his mother and stepfather, at the tender age of 15, from a friend who came over to his Colorado house one day and remarked how his mom was such a "bitch".

Torey Adamcik and Brian Draper, both 16 at the time, took inspiration from the Wes Craven horror movie "Scream" when they stabbed and killed their high school classmate Cassie Jo Stoddart in small-town Idaho in 2006.

And film-maker Joshua Rofe was prompted to make a documentary about them and other American juvenile offenders sentenced to life imprisonment without parole when he met a judge from Florida at a friend's birthday party.

"He was clearly conflicted" about putting a 15-year-old killer of a taxi driver behind bars forever, said Rofe, whose disturbing film "Lost for Life" premieres Saturday at the American Film Institute's AFI Docs festival.

"The girl that he sentenced to life without parole had the same name as his daughter. She was about the same age, and he said he often wondered if there was a better option," he told AFP.

"That was it. Right there, I knew I wanted to make a film about that."

The US Supreme Court ruled in June 2012 that mandatory life without parole for a minor convicted of murder -- the law in 29 states at the time -- was a form of "cruel and unusual punishment" prohibited by the US Constitution.

But it affirmed that judges have the option to impose the harshest possible sentence short of execution on youths -- a sentence now being served by 2,570 "juvenile lifers," according to the American Civil Liberties Union.

"The United States stands alone worldwide in imposing sentences of life without parole on juveniles," the non-profit Sentencing Project stated in a report last year.

Its research found that a majority of juvenile lifers had witnessed violence in their homes and had been suspended or expelled from school at some point. Moreover, 80 percent of female lifers had been victims of sexual abuse.

"Lost for Life" takes viewers into some of America's toughest prisons to hear Ind, Adamcik, Draper and others discuss the crimes they committed and to consider if they should at least be entitled to apply for parole.

"I wanted people to basically have the experience that I had when I first heard about juvenile life without parole," Rofe told AFP in a telephone interview.

Some of the inmates he met exhibited genuine signs of remorse as they calmly recounted their crimes. Others refused to acknowledge responsibility for having taken another person's life.

"Some people change. Some people don't," said Rofe, who in 2005 directed an indie film, "Brooklyn Battery," about a juvenile sentenced to three years for accidentally killing a fellow student in a brawl.

"There are undoubtedly people who committed murder when they were 15 years old who will never be fit to walk among us," he said.

"And there are undoubtedly people who committed murder when they were 15 years old who, let's say for argument's sake, after 20 or 25 years (behind bars), deserve a shot before a parole board."

The oldest juvenile lifer Rofe encountered in his research has been in prison since 1953 for a murder he committed at the age of 15.

"I have to ask, what harm is an 80 year old man going to cause if he were to experience the last few years (of his life) living in his lawyer's house if he was to be released?"

Rofe, who filmed parts of "Lost for Life" with an iPhone and raised financing via Kickstarter, interviewed supporters and relatives of his subjects prior to going into super-maximum prisons to meet them himself.

"I don't go (into prison) with a list of questions," he said.

"I go in and connect with a human being and we have a conversation. I tell them about myself in the same way that they're telling me about themselves."

"I try to go and have as quote-unquote 'normal' an exchange with somebody as I possible can -- but obviously there's nothing really normal about this. We're talking about pretty unique circumstances."

Rofe's next project, "Swift Current," focuses on former National Hockey League player Sheldon Kennedy, whose revelations about childhood sexual abuse upended the world of Canadian youth ice hockey.

Kennedy's story, he said, was "Penn State 16 years before it happened" -- a reference to the 2011 sex abuse scandal that engulfed Pennsylvania State University's hallowed American football team.

With the spread of the Internet and the growing availability of better and cheaper video cameras, Rofe believes this is a golden age for documentary film-making.

"Oh, my god, absolutely," he said. "Everybody has access to cameras (and) there are more and more avenues for distribution. No-one can tell you no. Everybody has a chance to make something that can go viral."