NYC gunman Shane Tamura is seen on video talking about his high school football career in California after a game in September 2015.

The gunman who killed an NYPD officer and three other people in a Midtown building on Monday evening was carrying a note that offered clues into his possible motive, sources told The Post.

Adams told reporters that investigators believe Shane Tamura of Las Vegas entered the building with the intent to reach the NFL 's offices but accessed the wrong set of elevator banks. "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."

Shane Tamura, the 27-year-old shooter who fatally shot himself, was a former standout high school football player who claimed he suffered from CTE, the brain disease that has been found to be common in football. Chronic Traumatic Encephalopathy is a degenerative brain disease that occurs with repeated head injuries.

Investigators believe Shane Tamura, of Las Vegas, was trying to get to the NFL offices in the building Monday after shooting several people in the lobby.

Four people were killed, including a police officer, and one was seriously injured when a gunman opened fire inside an office building before killing himself.

The crazed gunman who killed four people in Monday’s Manhattan massacre thanked a documentary on CTE brain injuries and listed off the names of several prominent neuroscientists in his