New York City Mayor Eric Adams said Tuesday that the gunman behind Monday’s deadly mass shooting in Midtown Manhattan targeted the headquarters of the National Football League (NFL), blaming the organization for brain injuries he believed he suffered while playing football.

The shooter, identified as 27-year-old Shane Tamura of Las Vegas, left a note suggesting he had chronic traumatic encephalopathy (CTE) — a degenerative brain condition linked to repeated head trauma in contact sports.

“The note alluded to that he felt he had CTE, a known brain injury for those who participate in contact sports,” Adams told CBS News. “He appeared to have blamed the NFL for his injury.”

Police believe Tamura entered the Manhattan skyscraper where the NFL’s offices are located carrying an AR-15-style rifle. Surveillance footage shows him firing in the lobby before taking an elevator to the 33rd floor, where he opened fire again. Instead of reaching the NFL offices, Tamura arrived at Rudin Management, a real estate firm in the same building, killing three people there before taking his own life.

Among the victims was 36-year-old NYPD officer Didarul Islam, a Bangladeshi immigrant and father of two, whose wife is expecting their third child. Others killed include security guard Aland Etienne, Julia Hyman of Rudin Management, and Wesley LePatner, an executive at BlackRock investment firm.

The tragedy has reignited debate about gun violence and firearm access in the United States, as Tamura reportedly had a history of mental health struggles but was still able to obtain the weapon.

The NFL has previously faced numerous lawsuits over concussions and brain trauma among players. While the league denies a direct link between American football and conditions like CTE, it has paid out over $1 billion in concussion-related settlements.

Tamura never played professionally, but he reportedly suffered a serious head injury in high school that ended his football career. His note allegedly called for his brain to be examined posthumously to help research into CTE, which can only be diagnosed after death.

Following the shooting, NFL Commissioner Roger Goodell issued a memo saying there will be an “increased security presence” at the organization’s New York offices in the coming weeks.

With input from Al Jazeera