Investigators are examining why the gunman who killed four people before taking his own life at a New York City office tower targeted the National Football League and whether he suffered from a brain disease.

Police lined the streets of New York City to pay their respects to fellow officer Didarul Islam, who was among the four victims of a gunman’s rampage Monday at a Manhattan office tower. This is

Members of the New York Police Department paid tribute to officer Didarul Islam, one of the people killed in Monday's Manhattan shooting. Islam died Monday when a gunman opened fire inside a New York City office building.