The New York City office building that was the site of a mass shooting Monday had significant security measures in place.

Police have identified the man who they say entered a Midtown Manhattan office building wearing body armor and opened fire, killing four people with a high-powered rifle, as Shane Devon Tamura.

Shane Tamura, 27, who police said fatally shot four people Monday in the midtown skyscraper that houses the National Football League's headquarters, said in a suicide note that he believed he had chronic traumatic encephalopathy,

Shane Tamura, the gunman who killed an NYPD officer and three civilians before taking his own life inside a Midtown Manhattan Park Avenue skyscraper, left a second suicide note behind in his Las Vegas home,