Among their discoveries at the grisly crash site: A metal shank one of the hijackers used to take control of the airplane, a passport that identified one of the hijackers and the black box recording devices that spelled out in crucial and heartbreaking detail passengers' actions to bring down the plane after it became clear the hijackers were on a suicide mission.
FBI Special Agent John Larsen recalled his first thought upon reaching the crash site in an old coal field in rural Shanksville, Pa., asking himself, "Where's the plane?"
Larsen, now retired, and other FBI investigators came together Sunday at the site of the crash where a memorial and visitor's center are under construction, to recall the work.
"There would have been a huge debris field if it had been shot down, and there was none," said John Shea, former special agent in charge of the FBI's Pittsburgh field office at the time. "There was a hole filled with heavy plane parts, and the lighter parts of the aircraft and other material was carried by winds and ended up in trees."
The Philadelphia Inquirer said events at the site will continue Tuesday, including the annual ceremony remembering the victims, which will be attended by Vice President Joe Biden.
Celebrity Families of 2014 [PHOTOS]