Chronic traumatic encephalopathy is a degenerative brain disease caused by repeated head trauma. Here's why it's so dangerous

Officials say a man with a rifle killed an off-duty New York City police officer and three other people before taking his own life at a Manhattan office tower.

Shane Tamura, the Park Ave. gunman who killed an NYPD officer and three others before taking his own life, left behind a suicide note saying he suffered from CTE, a brain injury often linked to playing football, and was targeting the NFL — but took the wrong elevator, officials said Tuesday.