"Many of the investigators believe that some of the initial clues that were uncovered about the terrorists' identities and preparations, such as flight manuals, were meant to be found. A former high-level intelligence official told me, 'Whatever trail was left was left deliberately -- for the F.B.I. to chase'".
Seymour M. Hersh, Annals of National Security, "What Went Wrong," The New Yorker, October 8, 2001
Seymour Hersh is the Pulitzer prize-winning reporter who uncovered the Iraq prison torture scandal and the My Lai massacre in Vietnam.
Why would evidence be planted? Obviously, to create a false impression that someone else committed the crime.