After killing four and taking his own life, Shane Tamura — a former football player at two L.A.-area high schools — left behind a suicide note, authorities say, alleging he had the brain disease CTE.

CTE is defined as a progressive neurodegenerative disease that causes brain damage similar to what’s seen in patients with Alzheimer’s disease, per the National Institutes of Health (NIH). There is a known link between repeated head trauma, contact sports, and the development of CTE.

The man who killed four people in the New York City skyscraper that is home to NFL headquarters carried a note blaming the league for his mental health problems.