I am looking for evidence of high-level government (FBI & Justice
Department in particular) blocks on investigations into hijackers and
other aspects of the 9/11 plot before the attacks. Most of the
information about this topic (that I've seen) comes from the testimony of 
U.S. attorney and former Chief Investigative Counsel for the U.S.
House Judiciary Committee David Schippers on talk shows, in
interviews, etc (which I already know about) - so I don't really know
if there actually is additional information out there to be located on
this topic. But if there is, I want it. I imagine that the most
information to be found on this topic will come from FBI
whistleblowers, so that might be a good place to look.

Any evidence, like authoritative testimony from people in the know,
that high-level government blocks were placed on investigations into
the 9/11 plot prior to the attacks is what I'm looking for.

Thanks.