Kathleen Peterson’s death first made national headlines after the premiere of The Staircase, a docuseries that followed the murder trial of Kathleen’s husband, Michael. The series’ first episodes debuted in 2004, and more followed in 2013 and 2018, as more developments in the twisty case arose. Now, a fictionalized version of the story, also titled The Staircase, is airing on HBO Max, starring Colin Firth as Michael and Toni Collette as Kathleen. The miniseries has sparked yet more debate about what did or didn’t happen to Kathleen.Kathleen died on the morning of December 9, 2001, at the bottom of a staircase in the couple’s home in Durham, North Carolina. Michael called 911, telling the dispatcher that his wife was unconscious and had had an accident: “My wife had an accident. She’s still breathing.” Soon enough, however, the death was being investigated as a murder, and Michael was the only suspect. Although there’s plenty of compelling evidence suggesting that Michael killed her — starting with the copious amount of blood that was found at the crime scene — he staunchly maintains his innocence. (He even accepted a lesser charge of manslaughter by taking an Alford plea, which allows a defendant to plead guilty without actually admitting guilt.) So what really caused Kathleen Peterson’s death? Below, the three major theories.Although Michael Peterson reported Kathleen’s death as an accident to the 911 dispatcher, when paramedics and police officers arrived at the Petersons’ home, they were alarmed by the volume of blood they found at the scene. Soon enough, Detective Art Holland and his team began treating the house as a potential crime scene, not the site of an accident. “When I first entered the house, I noticed an abundant amount of blood on [Kathleen], on the floor, on the walls, that just was not consistent with somebody falling down the steps,” Holland told ABC News. According to an NBC News report on Michael’s trial, investigators were particularly perturbed by the amount of dried blood they found at the scene, which indicated that time had passed between the incident and the call to 911.The autopsy report found lacerations on Kathleen’s skull, which, as district attorney Jim Hardin argued to ABC News, weren’t exactly consistent with a simple fall: “It’s impossible for me to believe ... that [this] could be caused from a series of missteps or a fall down 15 different stairs,” he said. “I just can't see that happening. This had to occur from multiple inflictions of blunt force trauma.” The autopsy also revealed that Kathleen’s brain had generated “red neurons” in the wake of its injuries — a finding that indicates cell death or a stroke. Hardin explained to NBC News that the presence of red neurons in Kathleen’s brain meant that she had to have been deprived of oxygen “for at least a two-hour period … She’s bleeding to death.”Later, during the trial, defense pathologist Jan Leetsma would argue that Kathleen could have aspirated, or coughed up, the blood at the scene before she died. But Dr. John Butts, who was then the chief medical examiner in North Carolina, dismissed this supposition, explaining that heavy aspiration of blood leaves blood behind in the lungs. That blood would be “visible grossly to the eye; if you don’t see anything there, if it’s not visible, then it can’t really be aspirated,” he testified. And there was hardly any blood in Kathleen’s lungs. He did, however, testify that “the wounds [to her skull] are consistent with inflicted trauma; beating would be one word that one could use.”Other details of the scene, and the body, raise questions about Michael Peterson’s claim of innocence. Michael said that he and Kathleen had spent the night watching a movie and drinking wine, leading to her stumbling, intoxicated and on valium, in her flip-flops on the staircase — but Kathleen’s blood alcohol content when she died was 0.07%, and the legal limit to drive is 0.08%, suggesting that she wasn’t as drunk as he had implied, although a possible interaction with valium could have heightened her level of intoxication. Fran Borden, a veteran police sergeant who worked the scene, testified that he was perplexed by the fact that Kathleen’s head and spine had remained in alignment even after she had supposedly fallen down the stairs. “I squatted in the stairwell and looked up the stairs, trying to visualize every possible scenario how this woman could have come down those stairs, landed in the position where she landed, and where did all that blood come from,” Borden testified in court. “It didn’t jibe. It didn’t fit.”After Michael was convicted in his first trial (he was later granted a second trial on appeal), Kathleen’s sisters, Lori Campbell and Candace Zamperini, told the Durham Herald-Sun that he had been a controlling, abusive husband who “flew into rages around the house.” They also alluded to a diary that Caitlin Atwater, Kathleen’s daughter from a first marriage, had kept during Kathleen and Michael’s marriage, in which she documented Michael’s abusive behavior.Despite compelling evidence to the contrary, Michael Peterson has adamantly insisted that he did not murder his wife ever since she died in 2001. As suspicious as the circumstances of her death may be, they’re not entirely clear-cut. NBC News describes how Michael’s defense attorney, David Rudolf, patterned his strategy for the first trial on O.J. Simpson’s trial: Rudolf focused on police and prosecutorial incompetence. For instance, the police allowed a large amount of traffic within the house even though it was being treated as a crime scene. Michael was allowed to embrace Kathleen’s dead body; his son, Todd, went into the kitchen after seeing the body to get a drink, and transferred blood there; neighbors had shown up at the house wanting to know what was going on. As Rudolf said at the trial, “The police didn’t tape off the area of the stairwell until 3:34 a.m., almost an hour after the call came in. And by then, it was just too late. The blood in that area had been completely altered, the scene at the house had been completely contaminated.”The prosecution had also focused on a missing blow poke — a two-pronged fireplace tool — that they argued was the probable murder weapon. But at the trial, Rudolf produced what he claimed was the Petersons’ blow poke, which he said had been found in their garage, and which wasn’t covered with blood. There’s no way to know whether that blow poke was the blow poke, but Rudolf also argued that Kathleen’s head injuries didn’t support the blow poke theory anyway. Although she had lacerations in her skull, she didn’t have skull fractures or brain swelling, which would be expected in a case of blunt force trauma. Forensic evidence in the case grew even more muddied in 2011, when Duane Deaver, an investigation analyst with the North Carolina State Bureau of Investigation who had worked on the Peterson murder case, was fired for tampering with and misreporting evidence in other cases.The question of Michael Peterson’s motive also remained fuzzy. At trial, the prosecution argued that Michael may have been motivated by financial woes: The Petersons owed $142,000 in credit card debt and were spending $100,000 more than they made per year, and Kathleen was worried about losing her job. She had a $1.8 million life insurance policy, of which Michael was the beneficiary. But according to Rudolf, the couple had resources of around $1 million and a house worth $600,000 to $700,000. Money made little sense as a motive.The prosecution also emphasized that Michael’s bisexuality might have caused a fight between the spouses, which they claimed led to a murder. In Michael’s trial, the prosecution submitted gay porn that he had viewed on the internet and his communications with a male escort as evidence that the marriage was troubled. Furthermore, Kathleen’s daughter Caitlin told NBC News that she didn’t believe that her mother knew about Michael’s bisexuality: “I genuinely cannot believe that she could know anything about that. I know — from every value that she’s taught me, from every — from the way that I was raised, that’s not something that she would have been willing to accept.” Michael, by contrast, said that Kathleen was “comfortable” with his sexuality in an interview in The Staircase.Even if Caitlin is correct, the prosecution’s theory of the case was highly speculative. Michael may have murdered Kathleen — there’s plenty of physical evidence that suggests her death wasn’t an accident — but the prosecution argued that on the night of her death, she found out about his bisexuality, and then they had a fight, and then he killed her, without any concrete evidence to back up their narrative.Another theory has taken hold of The Staircase viewers — along with medical and ornithological experts. It was first proposed by a neighbor of the Petersons, attorney T. Lawrence Pollard, who saw pictures of Kathleen’s head wounds and wondered whether there might be an alternate explanation for her death. After consulting with an ornithologist, he launched what has come to be known as The Owl Theory. Pollard, who subsequently joined Michael’s defense team, argued that a barred owl attacked Kathleen while she was on her porch hanging up Christmas decorations. The wounds were consistent with owl talons, and there was even a feather found in Kathleen’s hair. “I call it my smoking feather,” Pollard told Duke’s 9th Street Journal.Though the theory might sound far-fetched, it’s been supported by three credible experts, who petitioned the court to release information from Kathleen’s autopsy so that the theory could be more thoroughly investigated. The petitioners included Dr. Alan van Norman, a neurosurgeon and owl expert; Dr. Patrick T. Redig, a professor of veterinary medicine; and Kate P. Davis, the executive director of Raptors of the Rockies. All three wrote that Kathleen’s injuries were consistent with injuries from an owl’s talons, rather than from blunt force trauma: “The multiple wounds present suggest to me that an owl and Ms. Peterson somehow became entangled,” van Norman explained. “Perhaps the owl got tangled in her hair or perhaps she grabbed the owl’s foot.”“The lacerations on Mrs. Peterson’s scalp look very much like those made by a raptor’s talons, especially if she had forcibly torn the bird from the back of her head,” Davis wrote in the petition. “That would explain the feathers found in her hand and the many hairs pulled out by the root ball, broken or cut. The size and configuration of the lacerations could certainly indicate the feet of a Barred Owl.”Ultimately, the cause of Kathleen Peterson’s death died with her. As tempting as it can be to try to solve the mystery, there simply isn’t enough evidence to point conclusively in one direction or the other. That ambiguity is partly what keeps driving filmmakers, and viewers, back to the story year after year.