The Manhattan shooting gunman had a suicide note in his back pocket alleging that he suffered from CTE, a brain disease linked to head trauma, and expressed grievances with the National Football League, according to a source with knowledge of the investigation.

In the note, he also asked for his brain to be studied.

The gunman has been identified by police as 27-year-old Shane Devon Tamura from Las Vegas. Police said Tamura’s motives are still under investigation, and that he had “a documented mental health history.”

Tamura was a competitive football player in his youth, multiple sources told CNN.

The short note was scribbled over three pages and found by investigators after the shooting, the source said.

“Terry Long football gave me CTE and it caused me to drink a gallon of antifreeze,” the shooter wrote, according to the source. “You can’t go against the NFL, they’ll squash you.”

Terry Long, a former football player for the Pittsburgh Steelers, was diagnosed with CTE. He died by suicide after drinking antifreeze in 2005.

“Study my brain please I’m sorry Tell Rick I’m sorry for everything,” the note said, according to the source.

Some background: CTE, or chronic traumatic encephalopathy, is an Alzheimer’s-like disease has been most commonly associated with football players. Studies have shown that repetitive hits to the head — even without concussion — can result in CTE.

EDITOR’S NOTE: Help is available if you or someone you know is struggling with suicidal thoughts or mental health matters.
In the US: Call or text 988, the Suicide & Crisis Lifeline.
Globally: The International Association for Suicide Prevention and Befrienders Worldwide have contact information for crisis centers around the world.