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.

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 father of hero NYPD cop Didarul Islam grew so distraught when he learned of his son’s death at the hands of a deranged gunman in Midtown Manhattan that he suffered a stroke, his family told The Post.

Investigators are combing through evidence in the wake of New York City’s deadliest mass shooting since 2000, trying to figure out more about the gunman who opened fire Monday in Midtown, killing four people,

New York City shooter Shane Tamura targeted NFL Headquarters in a sick revenge plot a source has claimed, after a note was found offering clues behind his motive.

The Palmetto State Armory AR-15 assault rifle used in the shooting that killed an NYPD cop and four civilians is .223-caliber and is equipped with a black scope, a handguard and a shoulder sling.

The shooting happened at an office building with top financial firms and the National Football League. The shooter is dead, officials said.