Officials found a similar one at the Pennsylvania crash site:
FBI ends site work, says no bomb used
"The FBI said yesterday that it has finished its work at the crash scene of United Flight 93 after recovering about 95 percent of the downed airliner and concluding that explosives were not responsible for bringing it down.
FBI spokesman Bill Crowley said that the largest piece of plane recovered was a shred of fuselage skin that covered four windows -- a piece seven feet long from a jetliner that was 155 feet long." - Pittsburg Post-Gazette (09/25/01)