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.

A man carrying a high-powered rifle shot and killed four people, including an off-duty police officer working security in Midtown Manhattan, sources told ABC News.

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.