In 1972, three men in a Louisiana prison were placed in solitary confinement after a prison guard was murdered. Two of them are still there – even though many believe they are innocent

Angola prison, the state penitentiary of Louisiana, is the biggest prison in America. Built on the site of a former slave plantation, the 1,800-acre penal complex is home to more than 5,000 prisoners, the majority of whom will never walk the streets again as free men. Also known as the Farm, Angola took its name from the homeland of the slaves who used to work its fields, and in many ways still resembles a slave plantation today. Eighty per cent of the prisoners are African-Americans and, under the watchful eye of armed guards on horseback, they still work fields of sugar cane, cotton and corn, for up to 16 hours a day. "You've got to keep the inmates working all day so they're tired at night," says Warden Burl Cain, a committed evangelist who believes that the rehabilitation of convicts is only possible through Christian redemption.

Undoubtedly there is less violence and abuse among the prisoners under his wardenship than there was under his predecessors. But Angola is still a long way from being a "positive environment that promotes responsibility, goodness, and humanity", as he proclaims in the prison's mission statement. In fact at the heart of Cain's prison regime is an inhumanity that would make Jesus weep.

For more than 37 years, two prisoners, Herman Wallace and Albert Woodfox, have been locked down in Angola's maximum security Closed Cell Restricted (CCR) block – the longest period of solitary confinement in American prison history.

Having experienced the isolation of "23-hour bang-up" during my own 20 years of imprisonment, for offences of which I was guilty, I can attest to the mental impact that such conditions inflict. My first year was spent on a high-security landing where the cell doors were opened only briefly for meals and emptying of toilet buckets. If decent-minded prison officers were on duty we were allowed to walk the yard for 30 minutes a day. The rest of the time we were alone. The cells were 10ft x 5ft, with a chair, a table and a bed. You could walk up and down, run on the spot, stand still, or do push-ups and sit-ups – but sooner or later you had to just stop, and think.

As the days, weeks and months blur into one, without realising it you start to live completely inside your head. You dream about the past, in vivid detail – and fantasise about the future, for fantasies are all you have. You panic but it's no good "getting on the bell" – unless you're dying – and, even then, don't hope for a speedy response. I had a lot to think about. When the man in the cell above mine hanged himself I thought about that, a lot. I still do. You look at the bars on the high window and think how easy it would be to be free of all the thinking.

Such thoughts must have crossed the minds of Wallace and Woodfox more than once during their isolation. They are fed through the barred gates of their 9ft x 6ft cells and allowed only one hour of exercise every other day alone in a small caged yard. Their capacity for psychological endurance alone is noteworthy.

Wallace and Woodfox were confined to solitary after being convicted of murdering Angola prison guard Brent Miller in 1972. But the circumstances of their trial was so suspect that there are no doubts among their supporters that these men are innocent. Even Brent Miller's widow, Teenie Verret, has her reservations. "If they did not do this," she says, "and I believe that they didn't, they have been living a nightmare."

One man who understands the nightmare that Wallace and Woodfox are living more than anyone else is Robert King. King was also convicted of a murder in Angola in 1973, and was held in solitary alongside Wallace and Woodfox for 29 years, until his conviction was overturned in 2001 and he was freed. Together, King, Wallace and Woodfox have become known as the "Angola three".

The case of the Angola three first came to international attention following the campaigning efforts of the Body Shop founder and humanitarian Anita Roddick. Roddick heard about their plight from a young lawyer named Scott Fleming. Fleming was working as a prisoner advocate in the 1990s when he received a letter from Wallace asking for help. The human tragedy Fleming uncovered had the most profound effect on him. When he qualified as a lawyer, their case became his first. "I was born in 1973," he says. "I often think that for my entire life they have been in solitary."

Through Fleming, Roddick met King and then Woodfox in Angola. Their story, she said later, "made my blood run cold in my veins". Until her death in 2007 Roddick was a committed and passionate supporter of their cause. At her memorial service King played two taped messages from Wallace and Woodfox. In the congregation was film-maker Vadim Jean who had become good friends with Roddick and her husband Gordon during an earlier film project. "Anita's big thing was, 'Just do something,'" says Jean. "No matter how small an act of kindness. Listening to Herman and Albert's voices at her memorial was like having Anita's finger pointing at me and saying, 'Just do something'." And so he decided to make In the Land of the Free, a searing documentary, released later this month.

The story Jean's film tells is one that has resonance on many levels. All three men were from poor black neighbourhoods In New Orleans. They grew up fearing the police, who would regularly "clear the books" of crimes in the area, according to King, by pinning then on disaffected young black men. "If I saw the police, I used to run," King says. He admits to being involved in petty crime in his early years, but "nothing vicious". Eventually King was arrested for an armed robbery he says he did not commit and was sentenced to 35 years, which he began in New Orleans parish prison – and there he met Albert Woodfox.

Woodfox had also been sentenced for armed robbery – and given 50 years. On the day he was sentenced he escaped from the courthouse. He made his way to Harlem in New York, where he encountered the Black Panthers, the revolutionary African-American political movement. He witnessed the Panthers engaging with the community in a positive, constructive way, educating and informing people of their rights. He says it was the first time in his life that he had seen African-Americans exhibiting real pride, pride that emanated from the young activists, he says, "like a shimmering heatwave".

Two days later Woodfox was caught and taken to New York's Tombs prison where he saw first-hand the militant tactics of imprisoned Panthers who resisted their guards with organised protests. In Tombs, Woodfox was labelled "militant" and sent back to New Orleans where he joined King on the parish prison block, known – due to the high concentration of Panther activists – as "the Panther tier". There Woodfox became a member of the Black Panther party.

Outside, confrontations between the Panthers – described by FBI director J Edgar Hoover as "the greatest threat to the internal security of the country" – and the police were escalating. In an attempt to undermine the influence of the Panthers in New Orleans parish prison, officials tried to shoehorn men they termed "Black Gangsters" on to the tier – men like Wallace, also serving decades for armed robbery. One day Wallace was suffering from the pain of ill-fitting shoes. One of the Panthers, on his way to a court appearance, took his shoes off and handed them to Wallace. "Right then I knew that that was what I needed to be a part of," he says. In the summer of 1971 Wallace and Woodfox were shipped to Angola.

The civil rights bill had been signed in 1964, but seven years later Angola was still operating a segregated regime. Prisoner guards carried guns and were also responsible, according to well-documented sources, for organising systematic sexual abuse of vulnerable prisoners, which flourished in the prison's mostly dormitory accommodation. And violence between prisoners had reached such levels that Angola was known as "the bloodiest prison in America".

Woodfox and Wallace quickly extended the New Orleans chapter of the Black Panthers into Angola, establishing classes in political ideology and exposing injustices. They organised work stoppages, demonstrating to fellow prisoners the liberating power of acting with a "unity of purpose" and worked to eradicate the prevalent sexual abuses. But their political activities made them targets for the administrators. By the spring of 1972, tensions in the prison were dangerously high.

These were the conditions in which Brent Miller met his untimely death. That April, a prisoner work strike drew the attention of the guards who were called from normal duties to deal with the disturbance. Miller, a strong, athletic young man of 23, stayed behind alone. He entered a dormitory holding 90 prisoners and sat on an elderly prisoner's bed, drinking coffee and chatting. Moments later he was attacked and stabbed 32 times.

Two days later, four men identified as "black militants", including Wallace and Woodfox, were accused of the murder. It was quickly ascertained that one of the four had been inserted into the case by the prison administration. Charges against him were dropped. Another, Chester Jackson, admitted to holding Miller while the guard was stabbed to death. Jackson turned state's evidence in return for a plea to manslaughter. The case was tried in a town called St Francisville, the closest courthouse to Angola. The jury had been picked from the local populace, many of whom earned their living from the prison or had families and friends that worked there; all were white. Wallace and Woodfox were found guilty of Miller's murder, sentenced to life imprisonment without parole and taken from the court straight to Angola's CCR block to begin their life in isolation.

Robert King was brought to Angola from the parish prison two weeks after Miller's killing, as part of a roundup of black radicals. King had never met Miller and was in a prison 150 miles away when the murder took place. Yet he was investigated for the crime and identified as a "conspirator" before being transferred to lockdown on CCR alongside Wallace and Woodcock.

The following year a prisoner named August Kelly was murdered on King's CCR tier. A man named Grady Brewer admitted that he alone was responsible for the killing, which he said he carried out in self-defence. But King was also charged. The two men faced trial together in the same St Francisville courthouse where Wallace and Woodfox had been convicted the year before. The sole evidence against King came from flawed prisoner testimony. He and Brewer had not been allowed to speak to their attorneys for any length of time before their trial. When they protested, the judge ordered their hands to be shackled behind their backs and their mouths gagged with duct tape for the duration of their trial. The men were convicted and sentenced to life without parole. King later won an appeal; the federal court ruled that he had not been sufficiently unruly in the dock to warrant the shackling and gagging. He went back to trial in 1975, was re-convicted and immediately sent back to CCR.

When, after Scott Fleming's intervention in the case of Wallace and Woodfox in the 1990s, new lawyers reviewed the original trial of both men, discovering "obfuscation after obfuscation". The state had used a number of jailhouse informants against them, many of whom gave contradictory accounts of what they saw. One was registered blind. The key witness in the case was a man called Hezikiah Brown who testified he witnessed the murder. In his initial statement to investigators however, Brown said he had not seen anything. Three days later, when he was taken from his bunk at midnight by prison officials and promised his freedom if he testified, he agreed to say that he saw Wallace and Woodfox kill Miller. At the time Brown was serving life without parole for multiple rapes. Immediately after he agreed to testify he was given his own minimum security private house in the prison grounds and a weekly cigarette ration.

Wallace and Woodfox did not give up. They fought their convictions from their cells and in 1993 Woodfox was granted an appeal, forcing a new trial. The case was sent back to the same courthouse to be tried in front of a new grand jury. A local author, Anne Butler, who had published a book in which she detailed the case and was convinced that the right people had been convicted, acted as jury chairperson. No witnesses were called. Instead Butler was called upon to explain the case. Once again, the jury was composed of people who worked in Angola or were related to people who worked there. Butler's husband and co-author was Murray Henderson, who had been the warden of Angola when Brent Miller was murdered. It is worth noting that Henderson was a key member of the original investigation team and that, during that investigation, a bloody fingerprint was found close to Brent Miller's body. It was determined that it did not belong to Woodfox nor to Wallace, but despite the prison holding all the fingerprints of all the prisoners, no attempt was made to find out whose it was. The bloody print was also ignored at Woodfox's retrial. He was reconvicted and sent back to isolation in Angola's CCR.

It was 26 years before King won the right to another appeal. In 2001 the Federal court found that the jury in King's original trial had systematically excluded African-Americans and women and agreed that the case should be reheard. This time around the prisoner witnesses recanted and the federal court sent the case back to the district court for review. The state negotiated a deal with King. Reluctantly, and with his left hand raised instead of his right, he pleaded guilty to conspiracy; an hour and a half later he was freed.

In September 2008, Woodfox's conviction was overturned; the federal court ruled that his core constitutional rights had been violated at his original trial. Louisiana attorney general Buddy Caldwell could have set Woodfox free immediately. Instead he decided to contest the federal decision and Woodfox, now 64, was returned to Angola's CCR, where he remains. Herman Wallace, now 68, was moved to another Louisiana prison last year, where he too continues to be held in solitary confinement.

Today King, now 67, is still campaigning for justice for his friends. Albert Woodfox: "Our primary objective is that front gate. That is what we are struggling for and we are actually fighting for our freedom. We are fighting for people to understand that we were framed for a murder that we are totally, completely and actually innocent of." Robert King says he is free of Angola, but until his friends are free, "Angola will never be free of me."

Jean hopes his film will make a difference. "These men need help," he says. "Louisiana needs to be shamed into doing the right thing."

Further information: angola3.org. If you wish to help highlight the plight of the Angola 3, you can write to the Governor of Louisiana at the Office of the Governor, PO Box 94004, Baton Rouge, LA 70804, US.

In the Land of the Free is released on 26 March