Police have identified the man who they say entered a Midtown Manhattan office building wearing body armor and opened fire, killing four people with a high-powered rifle, as Shane Devon Tamura.

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.