On July 28, 2025, a 27-year-old gunman^[1] entered a New York City office building that is home to the National Football League’s headquarters. He shot and killed four people and injured one other before killing himself.

In a note found in his wallet, he claimed he had chronic traumatic encephalopathy, or CTE^[2], a degenerative brain condition believed to develop from repeated traumatic brain injuries. He asked experts to...