When Kathleen Peterson died in December 2001, it seemed at first that she had suffered a fatal fall down a set of stairs.
It wasn't hard to believe: The staircase was awkward and steep. And Kathleen's autopsy report indicated that the 48-year-old had a blood alcohol content of .07 percent, and that she had taken Valium.
But after medical examiner Dr. Deborah Radisch ruled Kathleen's death a homicide, investigators declared that the narrow stairway in Kathleen and her husband Michael Peterson's Durham, N.C. home became a crime scene.
Radisch wrote that Kathleen's death was caused by a "severe concussive injury of the brain caused by multiple blunt force impacts of the head. The number, severity, locations and orientation of these injuries are inconsistent with a fall down stairs; instead, they are indicative of multiple impacts received as a beating."
Lead detective Art Holland agreed with Radisch's report, telling ABC News in 2006 that there was "an abundant amount of blood on her, on the floor, on the walls, that just was not consistent with somebody falling down the steps."
And so an investigation began, setting off a chain of events that would forever change the lives of everyone who knew Kathleen, especially daughter, Caitlin Atwater, and four step-children, Todd and Clayton Peterson, and Martha and Margaret Ratliff. Together with Michael Peterson, all the kids agreed to appear in The Staircase, a documentary by French director Jean-Xavier de Lestrade, who was interested in learning about the American justice system.
But the 2004 documentary (and its 2018 sequel episodes) took on a life of its own. True-crime fans were fascinated by the mysterious nature of the case and the family's decision to support Michael, who police quickly suspected of being guilty. And it's now an HBO Max series starring Colin Firth as Michael and Toni Collette as Kathleen, along with an ensemble cast.
Michael's affairs are revealed
It was only natural for police to suspect Michael—the romantic partner is often the culprit in similar crimes—but Michael and Kathleen's blended family didn't believe that their father, a Vietnam vet and author, was capable of murdering the love of his life. "My mother and Mike had an absolutely loving relationship," Caitlin said when Michael was first arrested, as shown in the documentary. "And there is no way that either of them would wish any sort of harm on the other one."
But allegiances shifted when police discovered that Michael was bisexual and had engaged in affairs with men while married to Kathleen. In the documentary, prosecutors discussed finding photos of nude men on Michael's computer, along with explicit messages Michael had sent to a male escort before Kathleen died. The prosecutors said they believed Kathleen may have seen the message, leading to a fight on the night she died (and a murder motive).
And Michael admitted to having sex with men, but said that the affairs weren't romantic and that Kathleen knew about them. Michael's defense attorney David Rudolf was never able to prove that, though, telling documentarians that if Kathleen did know, she never told anybody else.
New details emerge about the death of Martha and Margaret's biological mother
While Caitlin and Kathleen's sisters, Candace Zamperini and Lori Campbell, soon turned against Michael, his adopted daughters Martha and Margaret Ratliff remained steadfast in the support—even after police began questioning their birth mother's cause of death.
Like Kathleen, Elizabeth Ratliff was found dead at the bottom of a staircase in her home in Germany, where her late husband and Michael were stationed in the military. Elizabeth died in 1985.
In the documentary, Michael remembers he and his then-wife, Patricia, had dinner at the Ratliff home and he stayed behind to help Elizabeth put Martha and Margaret to bed. He claimed that he went home without incident, only to learn the next morning that Elizabeth had suffered a stroke while walking up the stairs.
When Elizabeth's family found out that Kathleen had later died in similar circumstances, they told the detectives, who quickly decided the similarity between the deaths was no mere coincidence. As Det. Holland told NBC News in 2006, "Lightning don't strike the same place twice."
The police went on to request that Elizabeth's remains be exhumed and re-examined, which Martha and Margaret agreed to as they were convinced of their father's innocence. But, after studying the corpse, medical examiner Radisch said that Elizabeth's death was due to blunt force trauma, not a stroke.
Still, there were no definitive answers as to whether or not Elizabeth's death was an accident, leaving the jury to decide if they even wanted to consider the death in Germany as they considered their verdict in the North Carolina case. And ultimately, jurors felt "we had enough to deal with at 1810 Cedar Street and not to really talk about the death in Germany," one juror told NBC News.
Michael denied killing Elizabeth, telling the documentarians, "Liz was a very good friend of mine."
The supposed murder weapon is found and experts take the stand
There was also the matter of the murder weapon, which prosecutors suggested was a a fireplace tool called a blow poke that Kathleen's sister, Candace, had given to the family. It was a thin, hollow item that could cause the lacerations on the scalp, without causing a skull fracture. And it was missing.
Then, one day, Michael's son, Clayton, found the blow poke in the garage. It was undamaged and found not to have blood on it. Debate continued over whether or not it was even used.
Blood analysts took the stand and gave their takes on the crime scene. The prosecution's expert blood splatter analyst Duane Deaver told jurors that the patterns found in the stairwell were indicative of a brutal beating, but the defense's expert, Henry Lee, who testified in the O.J. Simpson trial, said that he believed the pattern suggested Kathleen had fallen and coughed up her own blood.
Again, jurors were divided on whose testimony to believe. A juror told NBC News, "I was probably more disappointed in [Lee] because he could not definitely conclude, without a shadow of a doubt on his part that this was not a beating."
"I never believed that the blow poke was the weapon," another added.
Michael is found guilty and later enters an Alford plea
Nearly two years after Kathleen died, a jury ruled that Michael was guilty of first-degree murder and judge Orlando Hudson sentenced him to life in prison without parole.
The Peterson family was devastated and worked tirelessly alongside Rudolf to clear their father's name. In between weddings and the births of Michael's grandchildren, they filed numerous appeals, but nothing stuck—until 2011, when it was discovered that the prosecution's blood splatter analyst had been fired for giving false evidence in at least 34 other cases.
Peterson was set free and the team began to prepare for yet another trial, only for Michael to decide to submit an Alford plea—this allows him to maintain his innocence while recognizing there's enough evidence for a jury to convict him—in 2017. As he told reporters at a media conference, "The odds are against you. You'll never win. Pick up your chips and go home."
He's since written two books about the trial and his experience in prison, titled Behind the Staircase and Beyond the Staircase.
To find out more about The Staircase, tune in to the documentary on Netflix. Additionally, all the episodes for the HBO max series are streaming now.
(A version of this story originally published on May 7 at 6 a.m. PT)