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.

“People were anxious but nobody panicked,” Gartner said, noting some people who worked in the building took the elevators down to the street, while others took the stairs. He didn't notice any smoke on the way down to the street, he said.