A suicide note blamed the NFL and possible CTE for his mental decline, though Tamura never played professionally, only in high school.

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, police sources said Tuesday.

The 27-year-old man who shot and killed four people at a Midtown office building on Monday carried a note in his pocket claiming he suffered from CTE, asked that his brain be studied and made references to the NFL.

Shane Tamura purposely shot himself in the chest to preserve his brain to be studied for the CTE he claimed to have.

Tamura was involuntarily held after being considered a danger to himself or others, according to a law-enforcement official.

Compared to what’s on the news, I don’t think any one of us, none of my teammates, sensed anything like this from Shane,” the former teammate told The Post.

Shane Tamura said he had CTE. His claim, which is unproven, has again put a spotlight on the NFL's troubling history with its players and the disease.

Terrifying scenes in Midtown Manhattan showed office workers running for their lives with their hands in the air. The surveillance image shows the suspect, identified as 27-year-old Shane Tamura, entering the skyscraper with an M-4 assault rifle at his side.