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.

The crashed helicopter atop 787 Seventh Ave. Photos: FDNY

Many office workers at 787 Seventh Ave. didn't know about the helicopter crash, which ultimately killed the pilot, until they left the building. Officials reported that nobody in the building was injured.

Hundreds of other building workers were also evacuated, including those at neighboring Sidley Austin, which has about 414 attorneys in its New York office. Other large tenants in the building include BNP Paribas and Stifel. As of early Monday evening, tenants were not clear when hey could return to the building, Gartner said.

“It's our understanding that the building has been successfully evacuated, along with all Sidley personnel. We executed our emergency protocols and are actively managing the situation,” a Sidley spokeswoman said.

Willkie has about 700 people, including attorneys and staff, in its New York office—all of whom were told to go home after the building evacuation, Gartner said. The firm's lawyers and staff will work remotely and at home until they can return to the office, he said. “They will make due, they'll find conference rooms at hotels or somewhere,” Gartner said.

Gartner himself had to immediately handle a client matter at 4 pm on Monday, just hours after the crash. “I went into my apartment and got on a board call with two other Willke Farr partners,” he said.

“It's business as usual,” he said about the firm's arrangements while working outside the office. “We're New Yorkers.”

Gartner said he's not anticipating a large interruption for Willkie's business, but he's not sure about the extent of the interruption until he knows when the firm can return to the office. The firm has 13 floors in the building, where it has been since 1998, he said.

Immediately after the building's evacuation, the streets were closed off in a block in each direction around the building. Dozens of firetrucks, ambulances and police vehicles were in the vicinity.

The top of the building was cloaked in fog amid the Monday afternoon rain showers. There was no visible smoke or fire outside the building around 3 pm.

A man who worked in the building's seventh floor and declined to give his name told ALM that the floor “rippled” when the helicopter hit the roof. The man, who was wearing a BNP Paribas vest, said he was told that he might be able to return to work Tuesday.

Gov. Andrew Cuomo, who spoke to reporters at the scene about 20 minutes after the crash, said the impact shook the building and caused a fire that was quickly brought under control, the Associated Press reported.

The crash, for some New Yorkers, evoked memories of the Sept. 11 attacks, though officials said there were no indications it was related to terrorism. One attorney tweeted that she was thinking of her former colleagues at Brown & Wood, a predecessor firm to Sidley Austin whose entire office was destroyed at the World Trade Center. “The firm moved to #787 after 9/11 and I'm looking forward to getting word from the firm that all are safe,” the attorney tweeted.

Willkie's Gartner said there was a sense of relief amid the building's workers. “It could have been much worse,” he said.