The gunman who killed fourin New York City had a generative brain disease linked to repeated head injuries playing sports, medical examiner's office said.

NYPD Crime Scene Unit personal leave the building at 345 Park Ave. in New York City where a gunman armed with a rifle opened fire in the lobby on July 29 in New York City. Shane Tamura, who killed four people before dying by suicide, had a generative brain disease linked to repeated head injuries while playing sports, the city's medical examiner's office said Friday. File Photo by John Angelillo/UPI UPI

Sept. 26 (UPI) -- The gunman who killed four people in a New York City office building housing the NFL had a generative brain disease linked to repeated head injuries while playing sports, the city's medical examiner's office said Friday.

On July 28, Shane Tamura, 27, killed himself after opening fire at 345 Park Ave. Tamura's note said: "Study my brain please. I'm sorry."

"Following a thorough assessment and extensive analysis by our neuropathology experts, OCME has found unambiguous diagnostic evidence of Chronic Traumatic Encephalopathy, also known as CTE, in the brain tissue of the decedent," the Office of Chief Medical Examiner said in a statement obtained by ABC News and other media outlets. "The findings correspond with the classification of low-stage CTE, according to current consensus criteria."

The report added: "CTE may be found in the brains of decedents with a history of repeated exposure to head trauma."

Pathologists, led by Chief Medical Examiner Dr. Jason Graham, didn't say whether CTE played a role in his behavior, including shooting people.

"We continue to grieve the senseless loss of lives, and our hearts remain with the victims' families and our dedicated employees," the NFL said in a statement.

"There is no justification for the horrific acts that took place. As the medical examiner notes 'the science around this condition continues to evolve, and the physical and mental manifestations of CTE remain under study.'"

His family didn't comment to The New York Times. They had said he suffered from migraines, mental illness and multiple concussions.

Tamura had two Mental Health Crisis Holds on his record, and a prior arrest for trespassing in Nevada, where he also received his concealed carry license.

Tamura, a former high school football player in California, wrote in the three-page note: "The League knowingly concealed the dangers to our brains to maximize profits."

The note was found in his pocket.

He drove from his apartment in Las Vegas to Manhattan. Authorities said he took the wrong elevator bank and instead reached the offices of Rudin Management, where he again opened fire after spraying bullets across the building's lobby.

NFL employees were warned during the incident to shelter in place, although the shooter never arrived in those offices.

Four people died in the shooting: NYPD officer Didarul Islam, Blackstone real estate executive Wesley LePatner and Rudin employee Julia Hyman, who was named by the New York Post.

In all, he fired 47 rounds, reloading once.

Researchers have studied cases of former athletes who played in contact and collision sports, including football players, ice hockey players and boxers.

"Being a high school football player is certainly a possibility for CTE," Dr. Ann McKee, the director of the Boston University CTE, told CBS News. "We have a recent study where we found about 30% of former high school players had CTE. Now that's a very select group of people. It doesn't mean 30% of the general population of high school football players have CTE, but it's a distinct possibility."

Researchers say the link is not conclusive because most families donate the brains because they were displaying the symptoms.

"There is damage to the frontal lobes, which can damage decision making and judgment,"McKe recently told The Times. "It can also cause impulsivity and rage behaviors, so it's possible that there's some connection between brain injury and these behaviors."

The center has studied thousands of brains of athletes, soldiers and others exposed to brain trauma.

In 2023, the center reported 90% of 376 deceased NFL players' brains examined were diagnosed with CTE.

Some former NFL players committed suicide, including Dave Duerson, who deliberately shot himself in the chest to preserve the brain. Duerson left a note asking to have his brain studied.

"I would never draw a direct line between someone's brain pathology and any specific violent act, because the majority of people who have CTE never committed anything like this," Dr. Daniel H. Daneshvar, chief of brain injury rehabilitation at Harvard Medical School, recently told The Times.

In 2024, the NFL allowed players to wear special head protection, called Guardian Caps.

Also, the NFL changed kickoffs to reduce full-speed tackles.

"They need to do much more than just the helmet design, which is never going to prevent CTE," McKee told CBS News. It's really rules of play and styles of play, eliminating the hits to the head that occur in practice as well as games, paying attention to the players, monitoring the players for the number of hits they've sustained and actually keep track of the players over time."