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

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

Workers in the area received shelter in place warnings on their iPhone during the melee. But Tamura’s senseless attack in the heart of the city's business district highlights the need to protect the rank and file even more,

The guard had been fatally shot as he crouched behind a desk to take cover as the gunman sprayed the building’s lobby with bullets, police said.

Investigators found the note in the wallet of 27-year-old shooter Shane Tamura who police said opened fire inside of 345 Park Avenue.

Shane Tamura, the Park Ave. gunman who killed an NYPD officer and three others before taking his own life, left behind a suicide note saying he suffered from CTE, a brain injury often linked to playing football, police sources said Tuesday.

The assault rifle used in mass shooter Shane Tamura’s deadly rampage at a NYC skyscraper was assembled piece by piece – with the critical part purchased by his boss, according to officials and sources.