The scene of the shooting in New York City at an office building that included the NFL’s offices. The shooter reportedly blamed CTE brain trauma for his actions. | Victor J. Blue/Bloomberg via Getty ImagesOn Monday evening in Midtown Manhattan, a 27-year-old gunman opened fire in an office building that houses NFL offices, killing four people, including a police officer and then himself. In a note left in his wallet, the shooter referenced chronic traumatic encephalopathy, commonly called by its initials CTE, and criticized the NFL for covering up the traumatic brain effects of playing football.“Study my brain please,” read the note left by the shooter, who was a former high school football player, according to the New York Times. The Times reported that the note also included references to NFL players who had also committed violent acts and were then diagnosed with CTE after their deaths. The shooter shot himself in the chest rather than in the head, following the example of retired NFL Hall of Famer Junior Seau. Seau’s suicide in 2012, which reportedly followed his own mood changes, helped bring the link between an NFL career and CTE to the public’s attention after his autopsy found evidence of the disease.More than a decade later, like many mental health conditions, CTE remains mysterious. The condition cannot be accurately diagnosed until after death and symptoms, such as trouble thinking clearly, can be hard to identify and easily confused with other neurological problems. We are still learning exactly how many people have CTE and exactly how much trauma is necessary for the disease to develop.Here’s what we know and what we don’t, as another tragedy puts CTE back in the news.If you or anyone you know is considering suicide or self-harm, or is anxious, depressed, upset, or needs to talk, there are people who want to help.In the US:Crisis Text Line: Text CRISIS to 741741 for free, confidential crisis counselingThe National Suicide Prevention Lifeline: 988The Trevor Project: 1-866-488-7386Outside the US:The International Association for Suicide Prevention lists a number of suicide hotlines by country. Click here to find them.Befrienders WorldwideWhat is CTE?Chronic traumatic encephalopathy is a degenerative brain disease, believed to be caused by repeated head injuries. Though more research on the condition is needed, scientists do not believe that a single head injury, such as a concussion, alone can cause CTE. In a person with CTE, nerve cells in their brain die, compromising mental functions. It was first described as “punch-drunk syndrome” in boxers in the 1920s, before attracting more attention in the 2000s after autopsies of former football players revealed the extent of their brain trauma. The death of Seau, an eight-time All-Pro linebacker, forced the NFL and the American public to take the toll on players’ brains more seriously, starting a debate that continues to this day. CTE is most commonly associated with professional athletes who play contact sports, especially football and hockey, but has also been associated with those serving in the military. Other people, particularly victims of physical abuse, can also be at an elevated risk — the only known risk factor is recurring brain trauma.What are the symptoms of CTE?It seems like a straightforward question, but scientists and public health providers still don’t have the full picture. Part of the challenge in recognizing CTE is that its symptoms are vague and can be confused with other forms of dementia, even Alzheimer’s disease in older patients. According to the Mayo Clinic, there are not specific behavioral or physical changes that are unique to CTE. But some of the commonly cited symptoms include:Difficulty thinking clearly and memory lossImpulsive and aggressive behaviorSudden onset of depression, emotional instability, or suicidal thoughtsTrouble walking and loss of control of bodily movementExperts think that younger patients with CTE are more likely to experience mood changes, while older patients endure more of the memory loss and are unable to think clearly, which can progress to more advanced dementia, according to Mayo.Patients are encouraged to get in touch with a health care professional if they experience any of those symptoms or if they suffer a serious head injury that could lead to brain damage.How do we diagnose CTE?Unfortunately, there’s no way to diagnose CTE in a living person. Currently, brain samples must be removed from the person and studied under a microscope — something that is not safe for a living patient. To diagnose CTE with certainty after a person’s death, an autopsy collects evidence of damaged brain tissue as well as the build-up of tau proteins that have also been associated with Alzheimer’s disease. This is obviously suboptimal: If we are ever to develop better interventions and treatments, we need to be able to diagnose people who are still alive with CTE. Scientists are working on new tests that would look for biomarkers in saliva or blood that could tell scientists that a person has the condition. New imaging technology could also provide a path to earlier CTE diagnosis.But for now, in living people, doctors can still attempt to screen for CTE as well as other similar conditions. If medical professionals suspect CTE, they will diagnose a patient with what is called traumatic encephalopathy syndrome, and provide behavioral or occupational therapy. That’s why doctors still urge people who worry they could have CTE to get checked out.There is also no specific treatment for CTE; in general, nerve cells that die in the brain cannot be revived, which means the damage done by CTE cannot be reversed through medicine. Does literally every NFL player have CTE?We don’t know — but it may be close. A 2023 study from Boston University’s CTE Center found that 345 of the 376 former NFL players whose brains they studied had the condition. Though the authors cautioned against interpreting that finding to mean 90 percent of players have CTE — the brains donated to brain banks may not be representative of all players — the research would seem to suggest that playing a sport that involves violently crashing into other people, even with helmets and padding, leads to a higher chance of having brain damage.Part of the mystery with CTE is it’s not clear why some people develop it and other people don’t. Repeated head injuries clearly put you at risk — a separate BU study of NHL players concluded that the risk of CTE increased by 34 percent for every season played — but they are not enough on their own. Scientists still have more to learn about what causes some people to develop CTE while other people do not.How common is CTE among the general public?Most of us, of course, do not play professional football or hockey. But that doesn’t mean we should ignore CTE’s risks. Some jobs, such as serving in the military or working in construction, can lead to frequent head injuries. New studies indicate even younger kids playing recreational sports face some danger of CTE. According to the Mayo Clinic, patients may also face a higher risk if they experience one head trauma and then another in quick succession, before their first injury can heal.One study published in JAMA Neurology in 2023 focused specifically on people who died before the age of 30 and were known to have repeated head injuries. In the study, the most common causes of death were suicide or drug overdose. The researchers found 40 percent of the people studied had at least a mild form of CTE; among those with the condition, 70 percent had played a variety of sports at the amateur level — in youth leagues, high school or college.Clearly, being an NFL lineman or NHL enforcer isn’t the only way to develop CTE. Concerns have been raised about the accumulations of the micro-traumas that occur when, for example, heading a ball in soccer. Volleyball players, too, can be concussed during game play, which at least opens up the possibility of CTE risk.The science on CTE is still evolving. But what should be clear after this week’s shooting in New York City is we need to make more progress quickly to prevent more tragedies.