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.

In a memo sent to staff on Monday evening, the Big Four consulting firm told staff to plan to work from home or another KPMG office on Tuesday.

A police officer was among the dead. The gunman carried an M4 rifle and died after shooting himself in the chest, police said.

The unidentified executive was shot and killed in the lobby of the ritzy Park Avenue office tower when gunman Shane Tamura opened fire.

A cross-country drive, a Midtown office rampage, and a note blaming football injuries. Here's what we know so far about the NYC shooter and his victims.

The 27-year-old man who shot and killed four people at a Midtown office building on Monday carried a note in his pocket, police sources told ABC News.