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.

A New York City office building where a gunman killed four people then himself has reopened a week after the deadly shootings. On Monday, workers trickled back into 345 Park Avenue in Midtown Manhattan, a skyscraper home to high-profile tenants including the NFL and asset management firm Blackstone.