This article explains...

Impossible to Prove a Falsehood True:

Aircraft Parts as a Clue to their Identity by George Nelson Colonel, USAF (ret.)

Notable Quotes:

"It will soon be five years since the tragic events of 9/11/01 unfolded, and still the general public has seen no physical evidence that should have been collected at each of the four crash sites, (a routine requirement during mandatory investigations of each and every major aircraft crash.) The National Transportation Safety Board has announced on its website that responsibility for the investigations and reports have been assigned to the Federal Bureau of Investigation, but there is no indication that mandatory investigations were ever conducted or that the reports of any investigations have been written."