NEW: NYC shooter Shane Tamura targeted NFL offices, left letter saying he suffered from CTE after high school football career

Shane Tamura's motive for walking into a Midtown Manhattan skyscraper on Monday evening and opening fire was due to grievances he had with the NFL. Tamura, who played high school football, left behind a suicide note saying that he suffered from CTE, or Chronic Traumatic Encephalopathy, which is a neurogenerative condition from repeated head injuries.

The note was found in Tamura's back pocket after he shot himself in the chest and died from those injuries. He asked for his brain to be studied.

"Terry Long football gave me CTE and it caused me to drink a gallon of antifreeze," said the note, written over three pages. "You can’t go against the NFL, they’ll squash you."

"Study my brain please I'm sorry Tell Rick I'm sorry for everything," said the note per a source to CNN. Terry Long played for the Pittsburgh Steelers and killed himself in 2005 after drinking antifreeze. It is unclear who "Rick" is.

Police say that Tamura, 27, did have "a documented mental health history." NFL player Junior Seau killed himself in 2012, asking for his brain to be studied. He was posthumously diagnosed with CTE.

Tamura was interviewed after a high school football win for Granada Hills in California.

NFL Commissioner Roger Goodell released a statement after the shooting, saying that an NFL employee was among those seriously injured by Tamura, who killed an NYPD officer, three others, and himself. That NFL employee is in stable condition, Goodell said.

"As has been widely reported, a gunman committed an unspeakable act of violence in our building at 345 Park Avenue," Goodell wrote. "One of our employees was seriously injured in this attack. He is currently in the hospital and in stable condition. NFL staff are at the hospital and we are supporting his family."

He went on to say that employees in New York should work remotely or take the day off and that there would be "an increased security presence" at the building.

The Blackstone building on 51st Street and Park Ave. houses NFL offices, as well as those of financial services companies. Tamura took his own life on the 33rd floor, in the offices of a realty company.

Tamura drove across the country with his rifle, arriving in New York City, and going straight to the Blackstone building. He could be seen on surveillance video striding confidently toward the building.

The NFL offices are on the 5th floor of that building, though Tamura ultimately went to the 33rd building, where he met his end at his own hands.