Right before a helicopter crashed on the roof of the AXA Equitable Center in Manhattan, office workers heard a loud buzzing from the helicopter—and then a big thump.

“It really shook the building,” said Steven Gartner, co-chairman of Willkie Farr & Gartner. Soon after, building staff told workers over the intercom that they were investigating. Minutes later, another message: They had had to leave immediately because the building was being evacuated.