Police are still investigating what exactly prompted a gunman to kill four people in a Manhattan office building yesterday evening, but perhaps the clearest aspect of his motive is the condition that he evokes in a note found on his body: chronic traumatic encephalopathy.The 27-year-old gunman, Shane Tamura, was a former high-school football player. He targeted the Midtown skyscraper that houses the National Football League, though none of the four people he shot and killed before ending his own life was an NFL employee. (According to a statement from NFL Commissioner Roger Goodell, one league employee was “seriously injured” and in stable condition at a hospital.) In his note, Tamura reportedly speculated that CTE might have been a cause of his mental illness, but it’s still too early for medical examiners to offer a diagnosis. (And even if an autopsy were to show anomalies in his brain, it could never reveal what precisely drove him to homicide.) Like at least one NFL player who died by suicide, Tamura asked that his brain be studied after he died.Concerns about CTE and football have been mounting for more than two decades. In 2013, the NFL settled a lawsuit brought by more than 4,500 former players who claimed that the league concealed from them the risks of brain injury, including CTE. CTE is both rare and difficult to diagnose, so scientists haven’t definitively established its symptoms. They’re thought to include memory loss, personality changes, suicidality, and loss of motor control—all of which can be both devastating and caused by any number of disorders. Research overwhelmingly validates the link between the condition and professional football careers.But the consequences of playing high-school football are not well studied—a major oversight, given that most people who play do not end up in the NFL, Eleanna Varangis, a University of Michigan professor who studies brain injury, told me. “The majority of the experience is at the youth level, and we still don’t know a lot about how those people look later in life,” she said.Because CTE can be diagnosed only after death, whether Tamura had it is not yet known. Clearly, based on his note, something appears to have led him to suspect that he did. But two experts I spoke with about the condition—both of whom are advocates for better research and care in understanding CTE—told me that they had little reason to suspect that a person like Tamura would in fact have had the condition just because he played high-school football. (So far, a high-school teammate and a coach have told NBC News that Tamura was a talented player, but no further details have emerged about his time in the sport.) Jesse Mez, an associate professor of neurology at Boston University’s Chobanian and Avedisian School of Medicine and a co-director of clinical research at its CTE Center, has studied the risks of CTE across football careers ranging from one year up to 30—high schoolers to professional players. He found that the longer people play, the greater their risk of developing CTE. After five and a half years of playing, the relationship starts to be linear. But in careers shorter than that, “the likelihood of getting the disease is quite low,” Mez told me.CTE is thought to be caused by repetitive blows to the head, whether or not they lead to concussions. But scientists have not been able to pin down the precise number of impacts (or concussions) that cause someone to tip over the threshold into CTE, and even if they could, the length of a player’s career is an imperfect proxy for how many times they hit their head, Kristen Dams-O’Connor, the director of Mount Sinai’s Brain Injury Research Center, told me. There is variability, too, in susceptibility: Some people might develop CTE after fewer blows to the head. Genetics may also play a role. Although research shows the chances that a high schooler would develop CTE are extraordinarily small, “it’s hard to say what small is,” Mez said.[Read: Tua Tagovailoa’s impossible choice]The ambiguities around diagnosing CTE are made only worse by the fact that it cannot be confirmed before death. “I think it would be a huge service to be able to diagnose it in life, even without absolute certainty,” Mez said. Some signs indicate that all of that fear and confusion has led to excessive self-diagnosis. CTE appears to be “uncommon” in professional football if you take all players into account, Dams-O’Connor told me. And yet, in a 2024 study of former NFL players, more than one-third believed they had CTE. To assume that CTE is to blame for, say, depression, just because a person played football, is “really harmful logic,” Dams-O’Connor said: It suggests that nothing can be done, that a person is doomed to a life of irreversible decline from a disease with no direct treatments. Whether or not Tamura had CTE, it’s chilling to think that his conjecture that it contributed to his mental illness may have driven him to violence.Plenty of other factors, football-related or not, may have also caused or exacerbated Tamura’s mental illness, Mez said. Some research suggests that high-school football players may have greater risk for comorbidities that affect brain health, such as cardiovascular or cerebrovascular disease. A 2018 study found that people who start playing football before age 12 are at risk of experiencing cognitive, behavioral, and mood-related problems earlier in life than those who start playing when they’re older. And studies have shown that brain injuries (from football or any other cause) are associated with mental-health issues, including a higher risk of suicide, homicide, and criminal tendencies. New York City Police Commissioner Jessica Tisch said at a press briefing last night that the gunman had a “documented mental-health history,” and multiple outlets have reported that he was placed on psychiatric hold in both 2022 and 2024.[Read: The future of detecting brain damage in football]The New York City Office of the Chief Medical Examiner said in a statement today that it would examine Tamura’s brain during an autopsy—just as he wished. Perhaps the findings will add to the messy, ongoing science of CTE. Or perhaps they will prove a lesson in what happens when players are too quick to suspect it.