The man suspected of shooting dead four people midtown Manhattan last month had years earlier threatened to kill himself as he dealt with depression and concussion symptoms, according to a phone call his mother made to police.

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.