(Photo credit: Tork Mason / USA TODAY NETWORK via Imagn Images)

The man who police say shot and killed four people and seriously injured another in a Midtown Manhattan office building Monday evening apparently held a grudge against the National Football League.

New York Mayor Eric Adams said in interviews Tuesday morning that the suspected gunman, 27-year-old Shane Devon Tamura of Las Vegas, left a note in which he said he suffered from chronic traumatic encephalopathy after playing high school football.

'He seemed to have blamed the NFL,' Adams said. 'The NFL headquarters was located in the building, and he mistakenly went up the wrong elevator bank.'

The note was found on his body after he reportedly died by suicide. It has not been confirmed as to whether he suffered from CTE, the degenerative brain disease that has been linked to contact sports, including football.

Among the four killed was an off-duty New York City police officer, 36-year-old Didarul Islam, working a private security detail. Surveillance video shows Tamura walking into the building carrying an assault rifle around 6:30 p.m., and three of the people killed were in the lobby.

Despite the alleged shooter not finding the NFL offices, one league official was shot and seriously injured, commissioner Roger Goodell said in a memo issued to employees Monday night. Leading financial firms also have space in the office building at 345 Park Ave.

'He is currently in the hospital and in stable condition,' Goodell wrote about the employee in the memo, which was obtained by ESPN. 'NFL staff are at the hospital and we are supporting his family. We believe that all of our employees are otherwise safe and accounted for, and the building has nearly been cleared.'

Goodell said the league office would have heightened security in the days and weeks ahead and told New York-based employees either to work remotely on Tuesday or take the day off.

Authorities said Tamura had a concealed firearms permit issued by the Las Vegas Police Department. He also previously was given a work card, now expired, by the Nevada Private Investigators Licensing Board.

ABC News reported that Tamura played running back for Grenada Hills Charter in high school in the Los Angeles area.

New York Police commissioner Jessica Tisch said Monday that Tamura had a 'documented mental health history.' He is believed to have arrived in New York shortly before the shooting after driving from Las Vegas. Officers found a revolver, a backpack, ammunition and unspecified medication in his vehicle, a double-parked BMW, per ABC News.

--Field Level Media