On 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.

In the US:

Crisis Text Line: Text CRISIS to 741741 for free, confidential crisis counseling

The National Suicide Prevention Lifeline: 988

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