Manhattan shooting suspect claimed to have CTE, mentioned NFL in note, sources say

Monday's shooting was at 345 Park Ave., which houses the NFL's headquarters.

The 27-year-old man who allegedly shot and killed four people at a Midtown office building on Monday carried a note in his pocket claiming he suffered from CTE and asking that his brain be studied, police sources told ABC News.

The note also made references to the National Football League, police said. The shooting on Monday took place at 345 Park Ave., which houses, among other companies, the NFL's headquarters.

CTE, or chronic traumatic encephalopathy, is a brain disease linked to repeated hits to the head, often seen in military veterans and athletes including football players, hockey players and boxers. CTE can’t be diagnosed in a living person with certainty, but doctors may suspect it based on symptoms and history of head trauma.

The suspect, Shane Tamura, who the police said has a documented mental health history, played high school football.

Tamura is alleged to have shot and killed four people, including an off-duty police officer working security in a Midtown Manhattan office building, officials said during a press conference Monday evening.

Police have not detailed a potential motive for the killings.

NFL Commissioner Roger Goodell sent a message to staff, saying one of the league's employees was also injured in the attack.

Law enforcement sources said Tamura had two Mental Health Crisis Holds in his background in Nevada, one in 2022 and the other in 2024. Those holds typically allow a person to be detained for up to 72 hours if they are thought to be a danger to themselves or others.

According to one source, when there is this type of hold in Nevada, officers have to transport the person to the hospital. Once that happens, medical staff take over and decide how long to hold the person.

A Las Vegas police source said Tamura also got his CCW, or concealed carry, in 2022 and also has a previous arrest for trespassing in the state.

The three-page note that was on Tamura was described on Tuesday by sources as rambling. It contained references to the NFL that sources described as vague.

The only way to confirm CTE is through an autopsy by specialists who look for specific changes in the brain. Researchers analyze the brains post-mortem to look for tau protein changes that define CTE.

The disease causes symptoms -- which usually appear years after the head trauma -- including memory loss, mood changes, confusion, and trouble thinking clearly.

This is a developing story. Please check back for updates.

ABC News' Darren Reynolds and Liz Neporent contributed to this report.