Despite initial speculation suggesting otherwise, the NFL’s headquarters in Manhattan now appear to have been the intended target in Monday evening’s attack on a Park Avenue office block.

Four people were killed and another seriously injured when a lone assailant opened fire in what authorities claim was a premeditated attack, apparently motivated by the gunman’s belief that he had suffered brain injury from a football-related head trauma.

The body of 27-year old Shane Devon Tamura was found with a handwritten note that said he believed that he had chronic traumatic encephalopathy (CTE), the degenerative brain condition linked to repeated concussions and traumatic brain injuries sustained by contact sport athletes. The note went on to accuse the league of ‘covering up the truth’, underlining the likely motive for the attack. Tamura, originally from northern California, had no criminal record but had been treated in the past for undisclosed mental health issues. He did not play college or professional football, but did so at high school and is reported to have harboured long-standing concerns over the impact of head injuries he believed he suffered in that time.

Although the building at 345 Park Avenue has high security protocols, including electronic entry checkpoints and private guards, Tamura gained access through the public lobby during evening hours when many offices were closing for the day. Officials are now reviewing footage and security measures after CCTV footage shows Tamura entering the lobby, openly carrying a legally-purchased AR-15-style rifle and wearing tactical body armour.

Most of the shooting occurred in the shared lobby, several floors below those which house the NFL’s offices. Tamura then entered an elevator and ascended to the 33rd floor, where he fatally shot a female Rudin employee before turning the weapon on himself. Authorities, however, believe Tamura took the wrong elevator bank, which prevented him from reaching the floors occupied by the league and potentially averted a longer casualty list.

The victims were identified as Didarul Islam, a retired NYPD officer working private security; Aland Etienne, a member of the building’s security team; Wesley LePatner, a Blackstone executive; and the unnamed Rudin employee Tamura encountered when exiting the elevator on the 33rd floor. An NFL employee was also shot and is currently in critical condition in hospital. Their identity has not been released.

“We lost four innocent New Yorkers tonight because someone believed a conspiracy and had access to a weapon of war,” mayor Eric Adams told the media during a late-night press conference, calling the attack ‘a targeted act of violence rooted in delusion and despair’ before praising the ‘swift, professional’ actions of first responders.

According to records, the weapon used in the shooting was legally purchased in Nevada, while a second was found in Tamura’s vehicle, along with handwritten notes and other materials relating to his grievances to the NFL and CTE. One document also referred to former NFL player Terry Long, who died by suicide in 2005 and was posthumously diagnosed with CTE.

Tamura reportedly requested that his brain be donated for scientific study. The NYPD confirmed that this will be honoured, although medical experts warn against drawing any conclusions about a diagnosis until a full post-mortem is completed.

NFL commissioner Roger Goodell issued a statement on Tuesday offering condolences to the victims and confirming that one league employee had been seriously wounded. The NFL’s Park Avenue offices will remain closed ‘until further notice’.

“This was an unthinkable act,” Goodell wrote in a letter to NFL employees. “But we stand together in grief, and in support of one another.”

The Park Avenue incident is believed to be the first mass shooting in the United States directly linked to grievances over sports-related brain trauma, although CTE has previously been associated with suicides and domestic violence involving former professional athletes.