The gunman who shot and killed an NYPD officer and three other people in midtown Manhattan on Monday carried a handwritten note claiming he had CTE — a degenerative brain disease that is the result of repetitive hits to the head, officials said.

Although the disease can only be diagnosed after an autopsy, Shane Tamura, a 27-year-old from Nevada who played high school football, wrote that football "gave me CTE." Here's what to know about the disease.

What is CTE?

CTE stands for chronic traumatic encephalopathy, a brain disease found in athletes, first responders, members of the military and other people who have had a history of repetitive head impacts, according to the CTE Center at Boston University . Examples include heading the ball in soccer and checking in ice hockey. The contin

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