Shane Tamura, 27, had the rambling writings stashed in his wallet when he stormed the Midtown skyscraper at 345 Park Ave., which is home to the NFL’s corporate headquarters, the sources said.

Tamura, who claimed he suffered from CTE, had scrawled “frontline documentary” on one of the pages — an apparent reference to the “League of Denial” doc that probed the links between the NFL and the brain injury linked to head trauma.

The note also made mention of the “Fainaru brothers” — the two ESPN reporters who co-wrote the “League of Denial” book.

The names of several doctors were featured on one page — including Dr. Ann McKee, who is the Chief of Neuropathology at BU Alzheimer’s Disease Research Center, and Dr. Christopher Nowinski, the co-founder of Boston University’s CTE Center.

Elsewhere in the ramblings, Tamura blamed football for his apparent struggle with CTE – the neurodegenerative disease chronic traumatic encephalopathy and begged for his brain to be studied in the wake of the massacre.

He also name-checked Terry Long, the former Pittsburgh Steelers player who was diagnosed with CTE after downing antifreeze to kill himself 20 years ago.

“Terry Long, football gave me CTE and it caused me to drink a gallon of antifreeze,” Tamura’s note stated, according to sources.

“You can’t go against the NFL, they’ll squash you.”

“Please study brain for CTE. I’m sorry. The league knowingly concealed the dangers to our brains to maximize profits,” he continued. “They failed us.”

The degenerative brain disease has been linked to concussions and other repeated head trauma common in contact sports like football, but it can only be diagnosed after someone has died.

Tamura also offered up an apology in the note, writing: “Tell Rick I’m sorry for everything.”

Law enforcement sources said that reference was apparently a nod to the person who purchased the rifle used in the attack.

The alarming revelations came as Mayor Eric Adams confirmed Tuesday that the mentally ill shooter appeared to have been targeting NFL’s corporate headquarters, which has offices in the Park Avenue high-rise.

Law enforcement sources said that reference was apparently a nod to the person who purchased the rifle used in the attack.

“We’re still going through the suicide note to zero on in the exact reason but at this time it appears as if it’s something attached to his belief he experienced CTE from the NFL,” Adams said in interviews.

Despite suggesting he suffered from CTE from playing in the NFL, Tamura only played high school football in California.

In the wake of the bloodshed, NYPD Commissioner Jessica Tisch said Tamura, who most recently lived in Las Vegas, had a known mental health history.

He is believed to have suffered at least two mental health incidents in 2022 and 2024, sources said.

Investigators are still probing an exact motive for Monday’s bloodshed.

They believe Tamura had been trying to get to the NFL offices on the building’s lower floors when he stormed into the lobby and opened fire, killing four people.

Chilling surveillance video captured the gunman exiting a double-parked BMW early Monday evening with a rifle and then calming marching into the building, which is also home to investment firm, Blackstone.