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 League, police said.

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.

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

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

The shooting on Monday took place at 345 Park Ave., which houses, among other companies, the NFL headquarters.

CTE, or chronic traumatic encephalopathy, is a degenerative brain disease linked to repeated blows to the head and concussions.

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

ABC News’ Darren Reynolds contributed to this report.