A stranger's lie saw Colin Warner spend 21 years in prison for a murder he didn't commit.

In 1980, Colin Warner was just 18 and living in a neighbourhood called Crown Heights in Brooklyn, New York.

He was a Trinidadian immigrant with dreadlocked hair and a core group of good, close, childhood friends around him.

On April 10, and on an otherwise average spring night, the cops came knocking. Arrested and snatched from the streets, his freedom was stolen, he was put behind bars. He wouldn’t see the outside world for another 21 years.

Murder, they told him. He had, apparently, shot 16-year-old Mario Hamilton dead. Colin Warner did not know who Mario Hamilton was. The police, however, did not believe this.

In the same neighbourhood, on that same April 10 spring day, Mario Hamilton’s brother Martell was standing outside his high school, when a young kid called Thomas Charlemagne rode up to him on his bike.

Charlemagne was just 14 – a young, eager-to-please, impressionable kid – who delivered Martell the news. His brother Mario had been shot, and he knew this, because he had seen the whole thing.

He actually hadn’t, but in the moment, he lied.

"I think he only told me that so that maybe trying to make me feel better. And trying to think that he's being helpful. But he only, instead of being helpful, ended up creating a disaster," Martell told This American Life in 2005.

It was a lie that would spiral with a kind of domino effect that would ruin lives, embroil innocent parties and take 21 years to fix.

Immediately, Martell Hamilton and Charlemagne were taken to the local police station. 14-year-old Charlemagne, having once lied about seeing the crime unfold, was taken into an interview room and grilled for hours. There was no adult around, no guardian in the room. Photo after photo was placed on a table in front of him, until time stretched too long and the lie grew too deep to crawl out of. He pointed to a photo. It was a photo of Colin Warner.

Despite being across town when the murder took place  - and with an alibi to prove this -  Colin Warner was arrested and charged with murder.

After Warner's arrest, word on the street was that another kid by the name of Norman Simmonds had pulled the trigger. On the day of the shooting, he told Martell he had intentions of killing young Mario. Six months after receiving this information, Simmonds was arrested. A year and a half after this, a trial finally began, with both Simmonds and Warner on trial for murder, despite both teens not knowing the other.

The prosecution had one key witness. It was Thomas Charlemagne.

The first trial saw a hung jury, the second saw both found guilty.

In 2005, Warner's lawyer Bruce Richenstrick told This American Life increasing racial tensions at the time of Warner's conviction, and a backdrop of increasing crime, played a role in his future.

"There was a pervasive, overriding fear in the city, I believe at that time. And I think that, to some degree, that might have had something to do with Colin Warner's conviction. At that time, a black man accused of murder, well, that's all the jury needed."

So, over the course of the next 20 or so years, Warner's childhood friend Carl King took the case into his own. He took a job as a court process server so he was able to meet more lawyers.

"I remember going in and always thinking about his feelings. Even if I was down in my personal situation I knew couldn’t tell him about it because I didn’t want him to be down too.

“We’d end and I’d be walking out one way and he’d be going the opposite way, back to his cell. And I always thought, ‘We need to be going the same way,'” King told the LA Times this year.

They chased down new witnesses. Worked hard. Eventually, they tracked down Simmonds, who, in being sentenced as a juvenile, was released before Warner. They convinced him to testify and admit to the killing.

On February 1, 2001, after serving 21 years in prison for a murder he did not commit, The New York Times reported a then 39-year-old would walk free.

At the time, Martell Hamilton told the Times the result was a tough one to hear.

"I'm very happy for the man," he said at the time. "He spent 21 years in jail for something he didn't do. On the other hand, it's kind of bittersweet. It brought back memories of how it was, and that my brother is gone and will never return."

Now, nearly 16 years after being released, the story of how Warner ended up incarcerated, and King fought for his freedom, has been made into a film called Crown Heights, which is to be released this weekend.