Hundreds of lawyers and staff at Sidley Austin and Willkie Farr & Gallagher returned to their Manhattan offices Thursday, three days after a helicopter crash-landed on the roof of the firms’ building, forcing an evacuation.

Both firms had been working outside their offices at 787 Seventh Ave. since Monday afternoon, when the crash occurred. Authorities were investigating the incident and its impact on the building, preventing tenants from returning the building Tuesday and Wednesday. The New York Fire Department said a fire broke out on the roof after the Monday crash, which killed the helicopter’s pilot.