High over the Pacific Ocean on Tuesday, the casing blew off one of the engines on United Airlines Flight 1175.

Passengers heard a loud bang and felt the plane shake violently. Those seated on the right side looked out their windows and saw pieces of metal flying. By the time the plane touched down safely in Honolulu around 40 minutes later, the engine was bare, its innards on full display.

It was not clear as of Tuesday evening what had caused the malfunction. Ian Gregor, a spokesman for the Federal Aviation Administration, said the agency was investigating.

Haley Ebert, who was in a window seat above the right wing, said she had raised her window shade after hearing a “huge bang” that she likened to a gunshot.