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.

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.

Workers returned to 345 Park Avenue in Midtown Manhattan under the watchful eyes of NYPD officers and a surplus of additional security. CBS News New York's Christina Fan reports.