Image copyright Nashville Police Department Image caption Cyntoia Brown was arrested in 2004, and jailed for life soon after

The tale of a 16-year-old killer is spreading around the world, after some of the planet's most famous women joined a campaign to free her.

Cyntoia Brown was sentenced to life in prison in 2004 for shooting dead Johnny Allen, a 43-year-old Nashville estate agent who picked her up for sex.

The teenager who went home with Mr Allen that night had been repeatedly raped and beaten. She was on the streets by order of her boyfriend, described as a violent, drug-addled pimp named "Kutthroat", who she said verbally abused her and made her strip at gunpoint. He had demanded that she make him some money.

The client who would become Cyntoia's victim showed her his gun collection, telling her he was a former Army sharpshooter. It made her nervous, she noted later.

Cyntoia ate dinner with him, watched some TV, and got into his bed. "He grabbed me in-between my legs - he just grabbed it real hard," she told the trial court. "I'm thinking he's going to hit me or do something like it..."

Then Allen turned over and reached to the side of the bed. Cyntoia said she panicked, thinking he was reaching for a gun. Convinced she was about to die, she said, she shot him in the head with a .40-calibre handgun that Kutthroat had given her.

Image copyright Kevork Djansezian/Getty Images Image caption Cyntoia had a .40 calibre handgun in her bag (file picture)

Cyntoia was tried as an adult, and pleaded self-defence. Prosecutors argued that she killed Allen to rob him, because she fled with his wallet and guns. The jury found her guilty of first-degree premeditated murder, first-degree felony murder, and aggravated robbery.

Now in her late 20s, she will first be eligible for parole at the age of 67.

'We have to do better!'

How does a girl's life get wrecked to this degree before she's even old enough to vote? And when a childhood has been blighted by horrendous abuse, what bearing should that have when the victim commits a grievous, irreversible crime?

These are the debates playing out on social media, since reality star Kim Kardashian West, singer Rihanna and supermodel Cara Delevigne all shared emotional posts under the hashtag #FreeCyntoiaBrown.

Twitter isn't ideal for conveying the complexity of this case. But that didn't stop Kardashian West's pro-Cyntoia summary garnering 379,000 likes.

"The system has failed," she tweeted. "It's heart-breaking to see a young girl sex trafficked then when she has the courage to fight back is jailed for life! We have to do better & do what's right. I've called my attorneys yesterday to see what can be done to fix this. #FreeCyntoiaBrown."

Image copyright Charley Gallay/Getty Images for LACMA Image caption Kim Kardashian West shared a strongly-worded post in Cyntoia Brown's defence

Singer Rihanna, the force behind 230 million record sales, was similarly moved.

"To each of you responsible for this child's sentence, I hope to God you don't have children," she wrote on Instagram.

Cara Delevigne shared the same post with her 40 million Instagram followers, writing, "The justice system is so backwards!!! This is completely insane."

The picture on all these posts seems to be from Me Facing Life: Cyntoia's Story - a powerful 2011 documentary by filmmaker Daniel Birman, who spent six or seven years charting Brown's case from the week of her arrest, throughout her trial, and after her conviction for murder.

His footage poses urgent questions about how the US handles young offenders.

Further, it shows how a dark cycle of abuse began with Cyntoia's grandmother, who says her daughter was the product of rape, and cemented itself with her mother Georgina Mitchell, who gave birth at 16, turned to alcohol and crack cocaine, and spent years in prison herself.

Jeff Burks, the Assistant District Attorney who successfully jailed Brown, has previously warned that being young and likeable should not exonerate her.

"She wasn't just somebody who make one mistake," he said. "She was a very dangerous person. The choices she made were hers. She's pretty and smart and articulate so people have decided to take up her cause. Let's not forget her crime."

While in prison, Cyntoia has mentored other women, and in 2016 gained an Associate's Degree from Lipscomb University's in-jail programme. She is now working on a further degree.

The case has already changed the law in Tennessee, which now recognises that there is no such thing as a child prostitute. If she stood trial today, Cyntoia would be treated as a victim of sex trafficking. Instead, she's in prison serving life as a sex worker who murdered her client.

Though some feel she earned her jail time, supporters are hoping the sudden surge in publicity will prompt a review of Cyntoia Brown's situation.

At time of writing, a petition seeking a presidential pardon for her has attracted over 186,000 signatures.