Midtown officer shooter Shane Tamura left behind a note when he died.The note criticized the NFL and mentioned CTE, Business Insider has learned.Tamura's note also referenced NFL player Terry Long.

The Midtown Manhattan corporate office shooter left behind a three-page handwritten note that criticized the NFL and said, "study my brain please" for the degenerative brain disease CTE, Business Insider has learned.

Investigators found the note inside the wallet of 27-year-old shooter Shane Tamura, who police said opened fire inside of 345 Park Avenue Monday evening, killing four people, including an NYPD officer and a Blackstone executive, before he turned the gun on himself. He died of a self-inflicted gunshot wound.

The first page of Tamura's note made references to CTE (chronic traumatic encephalopathy), asked "study my brain please," and said, "I'm sorry," said a law enforcement source who provided excerpts of the missive.

An excerpt from the second page of Tamura's note mentioned late NFL player Terry Long, who died by suicide in 2005 by drinking antifreeze. He had been diagnosed with CTE after he died.

"Football gave me CTE and it cause me to drink a gallon of antifreeze," Tamura's note read. It added, "You can't go against the NFL."

"They'll squash you," another part of the second page of the note said.

The note's third page read in part, "Please study brain for CTE."

"The league knowingly concealed the dangers to our brains to maximize profits. They failed us," the third page also read, referring to the NFL.

This story is developing and will be updated.