The crazed gunman who killed an NYPD officer and three other people in a Midtown office building on Monday evening, was carrying a note in his pocket that expressed grievances with the NFL, sources told The Post. Shane Tamura also claimed he suffered from CTE – a brain injury linked to head trauma, which he blamed football for. Tamura asked in the note that his brain be studied, according to the sources. The 27-year-old opened fire on Monday at a swanky skyscraper at 345 Park Ave, where the NFL has offices on floors five through eight. Tamura, who initially opened fire in the lobby, wound up taking the elevator up to the 33rd floor where he turned the gun on himself after the massacre. He shot himself in the chest, presumably to preserve his brain.