Players and coaches around the NFL reacted with shock and sadness Tuesday after a gunman killed four people at the New York office building housing the league's headquarters

The man who opened fire in a Midtown Manhattan office building, killing four people and wounding a fifth, appeared to be targeting the NFL headquarters and had a note claiming he suffered from CTE, police sources told ABC News.

The BU CTE Center is “willing to help” with the investigation into the NYC deadly shooting on Monday when the gunman allegedly targeted the NFL offices and claimed he had CTE. Shane Tamura, 27, who played high school football, killed four people inside a Manhattan skyscraper before he took his own life.