According to the Mayo Clinic, CTE — or chronic traumatic encephalopathy — is a brain disease most likely caused by repeated head injuries that damage the brain's nerve cells. The Mayo Clinic notes the only way to tell for sure if someone suffered from CTE is by autopsy.

Chronic traumatic encephalopathy, or CTE, is a brain disease linked to repeated hits to the head, often seen in military veterans and athletes.

Shane Devon Tamura — the man who killed at least four people in a New York City office building — claimed in a suicide noted that he suffered from chronic traumatic encephalopathy (CTE) while slamming the NFL. “Study my brain please,” Tamura, a former high-school football player, wrote in the note. "I'm sorry."