Shane D. Tamura, the man suspected of gunning down four people — including an NYPD officer — before taking his own life in a Manhattan office tower, left behind a haunting multi-page note that allegedly points to football and brain trauma as motives behind his deadly rampage.

Tamura, 27, was found dead from a self-inflicted gunshot wound on the 33rd floor of 345 Park Avenue, the very building where the National Football League’s headquarters are located. That chilling detail has fueled growing speculation that his attack was deliberately aimed at the NFL.

As first reported by the New York Post, investigators recovered a lengthy suicide note next to Tamura’s body and his M4-style assault rifle. The note, sources say, references chronic traumatic encephalopathy — or CTE — a degenerative brain disease linked to repeated head trauma, most commonly associated with football players.

Tamura, who starred as a high school running back in California, reportedly blamed football for his deteriorating mental state. “Terry Long football gave me CTE and it caused me to drink a gallon of antifreeze,” he wrote, referencing the late Pittsburgh Steelers player who also died by suicide and was diagnosed with CTE. “You can’t go against the NFL, they’ll squash you,” the note adds.

Tamura’s final message, described as several pages long, includes a plea for doctors to study his brain after death and an apology to someone identified only as “Rick.” CNN also confirmed portions of the note, which suggests Tamura felt overwhelmed by his mental decline and believed the NFL bore responsibility.

New York City Police Commissioner Jessica Tisch confirmed Monday that Tamura had a documented history of mental illness.

Though Tamura had long since left the sport behind — working most recently as a security guard in Las Vegas — he was once a standout on the field. During his senior year at Granada Hills Charter High School in Los Angeles, he played a pivotal role as a running back, earning six “Player of the Game” titles. One viral clip from 2015 shows local reporter Jack Pollon praising Tamura’s game-winning 69-yard touchdown in a victory over Kennedy High School.

Former head coach Walter Roby described him as a quiet, hardworking player who led by example. “He was one of my top offensive players,” Roby told FOX 11.

While questions still swirl around Tamura’s full motive, the reference to the NFL in his final words is igniting new scrutiny into how head injuries in contact sports may affect long-term mental health. CTE, which can only be confirmed through a postmortem brain exam, is linked to severe behavioral symptoms including aggression, impulse control issues, and depression.

As the investigation continues, authorities are working to piece together how a once-promising football talent spiraled into a mass killer — and whether his time on the field left damage that went far beyond the gridiron.