The gunman who killed four people inside a New York office this week before turning the gun on himself was carrying a handwritten note that said: "Study my brain."
Shane Tamura wrote that he had chronic traumatic encephalopathy (CTE) — a brain disease diagnosable only after death — investigators said.
But what exactly happened, and what is CTE?
Firstly, what happened in New York?
The 27-year-old man from Las Vegas killed four people before taking his own life on the 33rd floor of the Park Avenue skyscraper on Monday.
Officials said he intended to target the NFL headquarters but took the wrong elevator — instead spraying the lobby of a skyscraper and killing four people: a police officer, a security guard and two people who worked at companies in the building.
Police said Tamura had a history of mental illness, and the note found on his body suggested he had a grievance against the NFL over an unsubstantiated claim that he suffered from CTE.
What did his note say?
In the three-page note found on his body, the gunman accused the NFL of concealing the dangers of brain injuries linked to contact sports for profits.
In the note, Tamura "claimed to be suffering from CTE, possibly from playing high school football", New York Police Commissioner Tisch said.
"Study my brain. I'm sorry," Tamura was quoted as having written.
The note mentioned a 2013 Frontline documentary featuring former NFL players who suffered from CTE, which has no known treatment and can be caused by repeated shaking of the brain associated with playing contact sports.
What is CTE?
Chronic traumatic encephalopathy can affect regions of the brain involved with regulating behaviour and emotions.
This can lead to memory loss, depression, violent mood swings and other cognitive and behavioural issues, though researchers note that these symptoms can also be linked to other illnesses.
Experts say symptoms can arise years or decades after the last brain trauma.
Evidence of the disease has been found not just in those with long professional careers but in high school athletes as well.
So, did the gunman have CTE?
We don't know.
CTE can be diagnosed only by examining a brain after death, and it is unclear whether he was showing symptoms.
Tamura was never an NFL player but he did play high school football in California a decade ago and had a history of mental illness, police said without giving details.
The degenerative brain disease has been linked to concussions and other repeated head trauma common in contact sports such as football.
According to Boston University's Chronic Traumatic Encephalopathy Center, progressive degeneration of brain tissue in people with CTE includes the build-up of an abnormal protein called tau in a pattern that distinguishes it from other diseases, such as Alzheimer's.
Why is CTE associated with the NFL?
Researchers have established a connection between CTE and contact sports, military combat and other activities with repeated blows to the head.
After more than a decade of denial, the NFL conceded the link between football and CTE in 2016 testimony before Congress, and has so far paid more than $US1.4 billion ($2.1 billion) to retired players to settle concussion-related claims.
The pioneering efforts of forensic pathologist Bennet Omalu led to the diagnosis of CTE in Pittsburgh Steelers Hall of Fame centre Mike Webster — the first in a former NFL player.
CTE has been diagnosed in more than 100 former NFL players, including NFL Hall of Famers Ken Stabler, Frank Gifford, Junior Seau and most famously Aaron Hernandez, the former New England Patriots star who killed himself in prison at 27 years old while serving a life sentence for a 2013 murder.
Among nearly 2,000 former NFL players, one-third believed they were living with CTE, a 2024 Harvard paper concluded.
Who were the victims?
During the news conference, Commissioner Tisch said the death of Didarul Islam was "yet another reminder of everything you risk just by showing up to work".
The 36-year-old Bronx officer was the first of four killed in the attack.
He was an immigrant from Bangladesh who served as a police officer in New York City for three and a half years, working in a program that let private companies hire officers for security work.
He was married and had two young boys, and his wife was pregnant with their third child, Commissioner Tisch said.
Blackstone, one of the world's largest investment firms, confirmed one of its employees, Wesley LePatner, was also among those killed.
A Yale graduate, LePatner was a real estate executive, according to Blackstone's website, and she spent more than a decade at Goldman Sachs before joining the firm in 2014.
"She was brilliant, passionate, warm, generous, and deeply respected within our firm and beyond," the firm said in a statement.
A labour union identified the security officer killed as Aland Etienne.
His death "speaks to the sacrifice of security officers" who risked their lives to keep New Yorkers safe, Manny Pastreich, the president of Local 32BJ of the Service Employees International Union, said in a statement.
"Aland Etienne is a New York hero," Pastreich said. "We will remember him as such."
The last victim killed was Julia Hyman, a 2020 graduate of the Cornell Nolan School of Hotel Administration, who was working as an associate at Rudin Management, according to her alma mater.
NFL commissioner Roger Goodell said in a memo to staff that a league employee was seriously injured in the attack and was now stable at a hospital.
He said that "all of our employees are otherwise safe and accounted for".
AP/Reuters/ABC
