New York’s Office of the Chief Medical Examiner revealed the results Friday.

“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 findings correspond with the classification of low-stage CTE, according to current consensus criteria,” OCME said in a statement.

Tamura, a 27-year-old former high school football player, shot and killed Aland Etienne, Wesley LePatner, Julia Hyman and NYPD Det. Didraul Islam on July 28. He then shot himself in the chest.

He left handwritten notes, repeatedly referencing CTE and writing, “Study my brain please. I’m sorry.”

Though Tamura never played in the NFL, investigators believe he was targeting the NFL offices inside the office building where he killed four people.

“The League knowingly concealed the dangers to our brains to maximize profits,” Tamura wrote.

The three-page note found in Tamura’s pocket said that he wanted to have his brain donated to science so it could be researched, according to law enforcement sources.

Tamura’s family previously said he suffered from migraines and mental illness, as well as multiple concussions.

What is CTE?

Chronic traumatic encephalopathy, or CTE, is a brain disease that is most common in athletes who play contact sports, such as football players, ice hockey players and boxers. It is a degenerative disease, occurring after repeated head injuries cause cells in the brain to die. It can only be diagnosed in brain tissue after death.

Tamura’s final note also included the name of Chris Nowinski, a leading CTE expert.

“He played for 12 years. I think that’s one of the more important points, is that 12 years absolutely puts you at risk for developing CTE,” Nowinski said. “Low-stage CTE does represent significant changes to the brain. Brain cell death in the frontal lobes, which will affect judgment and behavior. There’s no question about that.”

The medical examiner’s office said “the physical and mental manifestations of CTE remain under study.”

Nowinski said the four victims of the shooting must remain the focus, but says this incident is a wake-up call.

“We shouldn’t look at this and say ‘this is why he did it,'” Nowinski said. “This hopefully is a wake-up call that we need to do more to prevent brain trauma in young athletes, because there are consequences for some of these people.”

What are the symptoms of CTE?

There can be a variety of symptoms of CTE, including behavioral changes like aggression and impulsivity, irritability and inattention.

“There are people who have been diagnosed with CTE who have engaged in violent acts. It is not accurate, I don’t think, to say that CTE causes violence,” said Dr. Kristen Dams-O’Connor, director of the Brain Injury Research Center at Mount Sinai.

While damage may begin in the frontal lobes of the brain, over time it can affect widespread regions, causing memory loss and dementia.

Symptoms of CTE do not typically appear right after someone receives a head injury, but rather develop as time goes on, according to the Mayo Clinic.

O’Connor said CTE is rare in people with limited head trauma exposure, but she stresses a major takeaway in this incident is that anyone experiencing symptoms of depression or anxiety, or having suicidal or homicidal thoughts should seek care.

CTE in the NFL

More cases of the disease have recently been found in former NFL players.

Aaron Hernandez, a former New England Patriots player and convicted murderer, was diagnosed with stage 3 CTE after his death.

Former NFL player and CBS Sports analyst Irv Cross was diagnosed with stage 4 CTE. His widow said that at the time of his death, he was seeing things.

The Boston University CTE Center said that out of 376 former NFL players it has tested, 345 had CTE.

The NFL has made changes in recent years to prevent head injuries, including new rules and equipment.

“The NFL does acknowledge CTE and will tell you the 100 things they’re doing to try to prevent it,” Nowinski said.

“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 Friday. “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’.”

Jenna DeAngelis contributed to this report.