NEW YORK -- A 27-year-old man wearing body armor and carrying an M4 assault rifle shot and killed four people, including off-duty New York Police Department Officer Didarul Islam, who was working security in a Midtown Manhattan office building, officials said during a press conference Monday evening.

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.

Investigators are combing through evidence in the wake of New York City’s deadliest mass shooting since 2000, trying to figure out more about the gunman who opened fire Monday in Midtown, killing four people,