True story behind The Staircase: The marriage of Michael and Kathleen Peterson, the sex worker and a

Colin Firth and Toni Collette star as the Petersons in new crime drama The Staircase. Photo / Supplied

It was one of America's most gripping murder trials of the last two decades.

At its heart was a seemingly perfect marriage. Yet in the early hours of a cold December morning in 2001 Kathleen Peterson would be found lying dead at the bottom of a flight of stairs, drenched in her own blood and with numerous deep lacerations to her head.

Kathleen's husband, author Michael Peterson, insisted his wife – addled by pills and booze – had tragically fallen to her death.

But the police were convinced that a simple fall could not have caused that level of injury. Kathleen, they were sure, had been bludgeoned to death by her husband.

A possible motive, the cops said, was that Michael had a secret sex life with other men. When Kathleen found out he went into a rage and killed his spouse with a fire poker given to the couple as a gift.

But the more prosecutors began to try and persuade the jury of this version of events during the 2003 trial, the shakier the evidence began to look.

Indeed, when one of Michael's potential trysts took the stand, he may as well have been a witness for the defence rather than the prosecution.

The notorious death of Kathleen Peterson at the age of 48 and the ensuing trial is the subject of a new drama, The Staircase. Now airing on Neon, it stars Toni Collette as Kathleen with Colin Firth taking on the role of Michael.

Watch the trailer for The Staircase here:

Michael Peterson, now 78, moved in with the then Kathleen Atwater in 1989, two years after the break-up of his first marriage.

The couple married in 1997 creating a very blended family. Their sprawling home housed not only Kathleen's daughter and Michael's two sons but additionally the two daughters of family friends George and Elizabeth Ratliff who had both tragically died some years before.

Michael made a living writing novels based on the Vietnam War as well as penning columns in a local newspaper. Kathleen was an executive with Nortel, a large Canadian telecoms firm.

Outwardly at least, the marriage was happy.

"Everywhere they went, people noticed Michael looking at Kathleen with the kind of pride that you just don't fake," Michael's solicitor Dave Rudolf said at his trial.

But they were under pressure. Nortel was imploding and would eventually go bankrupt. Kathleen had begun taking Valium to cope with the stress. Michael's books were not the runaway successes he'd hoped for.

What happened that night?

On that deadly night, Michael said he and his wife had been drinking and she'd taken some Valium tablets.

At 2.40am on December 9, Michael said he went into the house to find Kathleen at the foot the stairs after, he said, what must have been a terrible fall.

Police were immediately suspicious. There was blood everywhere and most curious was seven deep lacerations to Kathleen's head.

The autopsy report stated the death was caused by blunt force trauma. Kathleen had been beaten to death with a heavy object.

Detectives zeroed in quickly on what they suspected was the weapon.

Kathleen's sister Candace had given the family a blow poke, a tool similar to a fire poker. But despite searching, the police couldn't find the poker anywhere.

So, the theory went, Michael had used it to kill his wife and had then carefully hidden it. Only when that was done did he call the police.

Thousands of male pornographic images

But why would a seemingly happily married man beat his wife to death?

Police thought they had the answer to that one too.

At the 2003 trial, the prosecution alleged that Kathleen had found more than 2000 pornographic images of men on her husband's computer, reported US TV network NBC. More than that, she'd also found numerous emails between Michael and a sex worker called Brad where the sexual services he could offer had been discussed in detail.

In one of the emails sent to Brad, Michael wrote: "I used to pay to f*** a super-macho guy who played lacrosse."

"I'm very bi and that's all there is to it."

It was claimed that angry and ashamed at his secret being revealed, Michael murdered his wife.

Michael's sexuality no 'secret'

But under cross-examination, Brad's testimony didn't go entirely the prosecution's way.

He told the court plenty of the wives of the married men who were his clients knew of their husbands' sexuality and at the very least put up with the fact their various partner's had an itch that needed to be scratched.

Brad testified that Michael, unlike many of other married men he talked too, was happy to discuss his marriage and that he'd said that he had a "dynamite wife".

"Most clients don't want to say anything about the relationship.

"He said he had a warm relationship with his wife and nothing would ever destroy that."

The defence also argued that had Michael been seriously trying to hide his communication with Brad he would have tried harder to do so.

Yet he called Brad numerous times from the home phone, which was in Kathleen's name, with the number right there on the bill.

Furthermore, the defence said that while there was no evidence Kathleen knew about her husband being attracted to men, there was also no evidence that she did know and was upset about it either.

The idea that Michael's sexuality was inevitably some kind of sordid side of his life kept hush-hush was also thrown into doubt by his brother Bill Peterson.

"I've known about my brother's sexual orientation since I was a teenager," he told the jury, adding that it was no secret.

Although, Kathleen's daughter Caitlin testified that she "genuinely couldn't believe" her mother knew her husband slept with men.

During the trial, Michael's children publicly sided with his version of events of that night. But Caitlin did not.

Blow to blow poke theory

The prosecution's case also took a tumble when it came to the poker.

Police had said it had vanished, was likely the murder weapon and Michael had successfully hidden it out of reach.

As such, it was one of the more dramatic moments in the trial when the defence unveiled exhibit 280 – the blow poke.

It was in pristine condition, save dust collected over the years, with no dents or scratches that might suggest it had been used to beat someone to death.

And far from being hidden, it had been hanging in the Peterson's garage the whole time, overlooked by officers.

The defence hoped the holes in the case would be enough to sow reasonable doubt in the minds of the jurors.

And certainly the jury was split on a verdict, at least initially.

But talking to US TV station NBC, several jurors said it was the crime scene photos that were the clincher.

The injuries to Kathleen's head were so severe the jurors decided her death had to be malicious.

On October 10, 2003, Michael Peterson was sentenced to life in prison without the possibility of parole. He appealed twice, in 2006 and 2007, and lost both times.

However, in 2011, Peterson was released from prison pending a retrial following doubts about some of the expert evidence at the crime scene.

In 2017, Peterson pleaded guilty to the manslaughter of his wife but he did so with a key caveat. His plea acknowledged that there was enough evidence for him to be convicted but he nonetheless maintained his innocence, as he had done all along.

The greatly reduced sentence meant with time served he was a free man, more than 16 years after that fateful incident on the stairs. An incident the truth of which still no one has entirely solved.

• The Staircase is available to stream on Neon

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