During evening rush hour in New York City on Monday, a man calmly walked into a Park Avenue office building lobby and killed a police officer, then opened fire on other innocent strangers.

The New York City office building where a gunman fatally shot four people and himself reopened Monday, one week after the deadly rampage.Workers returned to 345 Park Ave., in Midtown Manhattan under the watchful eyes of NYPD officers and a surplus of additional security following the mass shooting.