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.

A 27-year-old man wearing body armor and carrying an M4 assault rifle shot and killed four people, including an off-duty police officer working security in a Midtown Manhattan office building, officials said during a press conference Monday evening.

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 unleashed an assault-style rifle in Midtown,