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.

Once a promising athlete, the 27-year-old faded from public view—until he showed up at 345 Park Avenue, gun in hand.

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

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.