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 shooting occurred in the lobby of 345 Park Avenue on East 52nd Street, an office building that records indicate is home to the NFL, Blackstone and KPMG.

A man carrying a high-powered rifle shot and killed four people, including an off-duty police officer working security in Midtown Manhattan, sources told ABC News.