The 27-year-old man who shot and killed four people at a Midtown office building on Monday carried a note in his pocket, police sources told ABC News.

The brain disease commonly known as CTE, caused by repeated head trauma, has prompted a long reckoning at all levels of football. The shooter at NFL headquarters believed he was suffering from it.

Authorities say the gunman who opened fire in a Midtown Manhattan office building on Monday night -- killing four people and injuring a fifth -- had a history of mental illness and may have been targeting the NFL.