Wearing body armor and wielding an assault-style rifle, the alleged gunman burst into a Midtown Manhattan office building shooting victims at random.

The gunman, who said in a note he believed he had CTE, targeted skyscraper that houses the NFL's headquarters.

Tamura was involuntarily held after being considered a danger to himself or others, according to a law-enforcement official.

The 27-year-old man who shot and killed four people at a Midtown office building on Monday carried a note in his pocket claiming he suffered from CTE, asked that his brain be studied and made references to the NFL.

The gunman in the deadly Midtown Manhattan shooting, Shane Tamura, wrote about the brain disease CTE in a note obtained by investigators.

One of the four victims gunned down by crazed Manhattan mass shooter Shane Tamura has been identified as a Cornell University grad who worked for the management company that owned the Midtown skyscraper,

The 27-year-old Nevada man opened fire at a New York City office building, killing at least four people and then himself.

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.

No motive has been announced. A suicide note was found expressing grievances with the NFL and saying Tamura suffered from CTE, a source said. Follow for live updates.