The 27-year-old Nevada man left a note at the scene voicing anger that his mental illness may be linked to his having played football, officials familiar with the investigation told NBC News.

A lone gunman walked into a midtown Manhattan office building Monday and opened fire, killing four people, including an NYPD officer, before turning the gun on himself.

Shane Tamura has been described as a 'great guy' and talented American football player by former friends and coaches, who have expressed their shock at the mass shooting