The Moreno brothers arrived at the building shortly before 10 a.m., their usual starting time. They took an elevator to the roof and began preparations for the day’s work. Yesterday may have been unusual for the men, investigators said. New cables connecting the scaffold to the track had been installed  or partially installed, investigators said. That was one of their open questions.

Major questions about the new cables remained unanswered, officials said. It was unclear, for example, who had installed them, and when. It was also not clear whether the installation had been adequately completed, or whether some work had been left to the window washers.

Investigators said there was no question that the brothers had not hooked themselves up to their safety harnesses, but it was not clear whether this was negligence or if the men had not had a chance to do so before the accident occurred.

Image Police officers removed the body of Edgar Moreno, who fell 47 floors. Alcides Moreno, his brother, was gravely injured. Credit... Marko Georgiev for The New York Times

One police official said the cables were adequately hooked up to the scaffold, but that the brothers apparently were trying to secure them solidly to the beam-track. Somehow, the official said, the cables seem to have come loose from the beam and may have whipsawed as the scaffolding fell, dragging the brothers over the side with it.

It all happened in a moment.

Pedro Nuñez, 32, who was working on the fifth floor of a building across East 66th Street, was a witness.

“I was painting, looking out the window, and I saw the scaffolding come down very fast,” he said. It was just a blur, he said, the scaffold falling so fast he could not make out the men falling with it, and he then heard a roar that resounded in the neighborhood.