A 27-year-old Cornell grad tragically landed in the Park Avenue killer’s crosshairs — because he accidentally took the wrong elevator to her employer’s floor and encountered her, shooting her dead.

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

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.

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 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.