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.

Young athletes (ages 6–14) in tackle football experience vastly more head impacts — up to 15 times more — and 23 times more high-magnitude impacts compared to flag football participants [3]. Even subconcussive hits that do not cause symptoms can lead to cumulative brain injury and are not prevented by helmets alone [4].