Workers returned to 345 Park Ave., on Monday​ under the watchful eyes of NYPD officers and a surplus of security after the NYC mass shooting.

Officials said the gunman fatally shot himself after his attack, which set off a chaotic scene that rippled out into the heart of Manhattan during rush hour.

The former high school football player left a note at the scene repeatedly mentioning “CTE” — chronic traumatic encephalopathy, a brain condition caused by injuries to the head.