BOSTON (AP) — The degenerative brain disease that has plagued the National Football League for two decades, leading to a billion-dollar lawsuit, congressional hearings, a high-profile movie, and a constant stream of former players' obituaries, has now violently intruded into America's favorite sport in an unprecedented way.

The Las Vegas casino worker who killed four people in a New York City skyscraper, home to the NFL’s headquarters, left a note blaming the league for his mental health issues.

Shane Tamura, 27, a former high school football player, left a three-page note in his wallet claiming he had chronic traumatic encephalopathy — a condition that can only be diagnosed after death — and urged those who discovered him to: “Study my brain.” His grievances against the NFL included allegations that the league prioritized profits over player safety by hiding the damage CTE and football could cause.

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In a chilling echo of a disturbing pattern among NFL players who have taken their own lives, Tamura shot himself in the chest, thus preserving his brain for an autopsy that could verify whether his self-diagnosis was accurate.

CTE, a progressive brain disease associated with repeated head injuries common in military combat and contact sports, has been identified in over 100 former NFL players and poses a serious threat to the United States' leading professional sports league.

The risks associated with CTE have led some states to consider banning youth football, prompted leagues at various levels to reduce contact during practices, and resulted in the implementation of concussion protocols and rule modifications to mitigate the sport's inherent violence.

Here is what we understand and what remains uncertain about the link between CTE, the NFL, and the shooter.

What is CTE?

Chronic traumatic encephalopathy affects areas of the brain that are involved in regulating behavior and emotions. This can result in memory loss, depression, violent mood swings, and other cognitive and behavioral problems. However, researchers caution that these symptoms can also be attributed to other conditions.

Experts explain that symptoms may emerge years or even decades after the last brain injury. The disease has been discovered not only in those with lengthy professional careers but also in high school athletes.

Why is it associated with the NFL?

Researchers have established a link between CTE and contact sports, military combat, and other activities involving repeated head impacts. After years of denial, the NFL acknowledged the connection between football and CTE in 2016 during congressional testimony and has paid over $1.4 billion to retired players to settle concussion-related lawsuits.

The 2015 film "Concussion," starring Will Smith, highlighted the pioneering work of forensic pathologist Bennet Omalu, who first diagnosed CTE in Pittsburgh Steelers Hall of Fame center Mike Webster. Hall of Famers Ken Stabler, Frank Gifford, and Junior Seau, as well as Aaron Hernandez, have also been diagnosed with CTE. In a 2017 study, evidence of the disease was found in 110 out of 111 former NFL players’ brains examined.

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