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.

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.

There were 23 shell casings and 13 bullet fragments recovered from the lobby of 345 Park Avenue and 24 spent shell casings on the 33rd floor, police said.