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.

Police say the gunman entered a large office building on Park Avenue with an assault rifle and opened fire, killing four people.

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.