Barriers were erected in front of the ritzy Midtown Manhattan office building where a crazed gunman killed four people Monday, with access being restricted even to some who work there.

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 stalked through a Manhattan office tower firing a rifle, killing four people, including a New York City police officer, and wounding a fifth before taking his own life, officials said.