Marcia Clark can never forget the pain, writes Celia Walden.

Angry isn't the right word, said Marcia Clark, after a brief pause. "What I feel now, still, is profound disappointment and pain. Because whatever you think about OJ Simpson's guilt, two innocent people were brutally murdered and the killer never brought to justice."

Today - as she was every day of the American football player's 1995 murder trial - Clark remains haunted by the words of his beaten wife, Nicole Brown Simpson: "He's going to kill me, and he'll get away with it because he's OJ Simpson."

As the lead prosecutor in the most publicised criminal trial in American history - a trial that was televised for 134 days, turning the prosecutors, defence attorneys and judge into soap stars and celebrities - Clark can move on but will never forget. She's not allowed to.

Twenty-one years on, there is still a morbid and microscopic interest in the case, inflamed by every suggestion of new evidence.

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This month, a knife "discovered" on OJ's former Brentwood estate was handed to police. "But it has been excluded, I believe," says Clark. "It was actually discovered years ago by a construction worker who gave it to a detective who took it in to police but was told 'the case is closed'. So he just turned it back in."

The critically acclaimed 10-part TV series, American Crime Story: The People v. OJ Simpson, which starred Cuba Gooding Jr, John Travolta, David Schwimmer and Sarah Paulson, has undoubtedly given the case yet another lease of life.

"My heart really sank when I heard the series was going to happen," admits Berkeley-born Clark, a warm 62-year-old now living just outside Los Angeles.

"I didn't want to relive this nightmare that was traumatising on every level. Because really I believed I was just seeing the justice system thwarted every single day. Two people were dead, and nobody seemed to care. The whole thing had just become a circus."

A circus that continued long after OJ was acquitted of murder - but found liable for the wrongful death of the second victim, Ron Goldman, in a civil suit two years later - and only ended with the former NFL star's 2008 conviction and imprisonment for an armed robbery and kidnap in Las Vegas.

When Clark was told that the series was to be produced by Ryan Murphy, however, her fears subsided.

Overall, the characterisation has been pretty accurate, she feels. A good defence lawyer will always push the envelope, Clark maintains of Johnnie Cochran, "and it's up to the judge to ensure the trial isn't hijacked by inflammatory subjects that have no real relevance to the issues.

Our judge did not do that, and so we had a trial awash with claims of racism and crackpot conspiracy theories. There was no reason to allow Cochran's use of racial epithets into the trial, for example. By doing so, the judge turned the trial into a referendum on race instead of a search for the truth".

Clark has been enjoying impressive sales figures for her reissued 1997 bestseller, Without A Doubt, since the series began. And the divorced single mother has finally emerged a heroine.

But it was the way she was treated inside the courtroom, she says, "not by the lawyers but by Judge Ito, that was the worst thing. He treated me like a second-class citizen, and a jury takes their cue from the judge".

So disgusted by the unanimous "not guilty" verdict of October 1995 that she gave up the law and became a virtual recluse for two years as she wrote her book.

Clark now handles court-appointed criminal appeals, with a lucrative sideline as a crime fiction writer.

She has changed her mind on several issues pertaining to the case, notably cameras in courtrooms. "If there hadn't been cameras in the trial, no one would know what a travesty of justice it was, at least in my eyes."

So does she believe OJ would have been convicted if he had been white?

"Yes. He would have. If it had been a famous football player, it's very likely he would have. But ultimately juries will believe what they want to believe."

This jury wanted to believe that OJ had been framed by a racist cop, Mark Furhman, and that the glove famously found at the crime scene was planted.

One thing it's hard to remain sanguine about is the conspiracy theorists. "We went to such great lengths to prove them all wrong," Clark sighs, eyes to heaven. "I've never before or since seen such damning physical evidence."

And despite OJ spectacularly failing a lie detector test, there was the conspiracy theorists' titbit of choice, the gloves, which, when tried on in court by the accused were too small. "It was the judge's idea," sighs Clark.

"I objected because Simpson had to have latex gloves underneath for evidence purposes, which would mess up the fit. Not only that, but have you ever tried to put gloves on a kid who doesn't want to put gloves on? They make it impossible."

Over two decades on, with OJ coming up for parole next year, Clark remains as incensed as she was in that courtroom.

"I'm sure he hasn't been a problem prisoner," she says wearily.

"He's older and disinclined to be problematic. Oh and I've heard that he gets really soft treatment in there and that he throws Super Bowl parties in his cell. Have you seen the musical Chicago?" she laughs mirthlessly. "Like that.

"But look: I gave [the case] every fibre of my being and although in the past I have said I felt guilty that the killer was never brought to justice, that implied that I didn't fight hard enough.

"No, what I really felt was horribly badly for the families I couldn't deliver justice to. And I always will."