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.

The Midtown Manhattan building that was the scene of a deadly shooting last week reopened Monday, but under very different circumstances. The building at 345 Park Ave. was once filled with the everyday buzz of office life,

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. New York City police officers and a police dog were seen guarding the building,