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

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

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,

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

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.

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.