The NFL is reeling after a shooting on Monday night in midtown Manhattan aimed at NFL headquarters. New York City Mayor Eric Adams said on Tuesday that the gunman, who killed four people and critically wounded a fifth, was attempting to target NFL headquarters but took the wrong elevator by mistake.

The shooter had a note in his pocket claiming that he suffered from chronic traumatic encephalopathy (CTE) and asking that his brain be studied.

Police said the note also made references to the NFL, per ABC News. The shooting took place at 345 Park Ave in New York, a building that houses NFL headquarters among other companies.

“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.”

CTE is a brain disease linked to repeated hits to the head. It is commonly found in military veterans and professional athletes, including professional football players. CTE cannot be diagnosed with certainty in a living person, but doctors can suspect a patient suffers from CTE based on symptoms and history of head injury.

The shooter, Shane Tamura, had a documented history of mental illness. He played football in high school in California nearly two decades ago, per ESPN.

Mayor Adams said that Tamura appeared to target NFL headquarters, but took the wrong elevator after entering the building.

“He, from our preliminary investigation, he took the wrong elevator bank up to the NFL headquarters,” Adams said. “Instead, it took him to Rudin Management, and that is where he carried out additional shootings and took the lives of additional employees.”

One NFL employee in stable condition after NFC shooting

One NFL employee, Craig Clementi, was struck in the back by a bullet during the shooting.

NFL Commissioner Roger Goodell released a memo to league employees, which ESPN obtained, following the attack. The memo confirmed that Clementi is in stable condition.

“One of our employees was seriously injured in this attack. He is currently in the hospital and in stable condition,” Goodell wrote in the memo. “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 added that the league will have an increased security presence at the headquarters offices in the future.

“Every one of you is a valued member of the NFL family,” Goodell wrote. “We will get through this together.”