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.

Twelve years after moving to the United States from Bangladesh, Didarul Islam sought to join the ranks of the nation's largest police department because he "got inspired" by the "blanket" of protection the NYPD provided to his community, according to an essay he wrote as part of his police academy application.

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.