Before the first trial - the conspiracy charges - it's not clear to me how DOJ could have helped? Either legally or otherwise. And it seems from the memos that they, Clark/DOJ, weren't sure what Garrison had. Hoover dismissed him - correctly - as a crank and fraud. Which he was. But Clark and DOJ appeared to be confused; he comes through on the phone calls with LBJ as to not knowing what Garrison had. And LBJ was caught between the two sides.
Here's LBJ in a call with Connally (March 1,1969) on the Garrison investigation: "They [the FBI] have heard these things [about Ferrie], and they interviewed 'em back in, for the Warren Commission. They do not give them any credit to it, but we can't ever be sure, and we just want to keep watchin' and so on and so forth..."
Remarkably, Connally had initiated the call to tell LBJ that a reporter told him Garrison had evidence that "three four assassination teams" (he meant people) had been sent from Cuba to assassinate JFK. And that two of them were Oswald and Shaw. LBJ told him that there was nothing to it. As we look back we have to realize there were all kinds of wild rumors and stories coming out at that time about Garrison and the assassination, e.g., Pearson's stories.
*After* the acquittal I can see how legally they could have stepped in. I.e., Shaw's civil rights were being denied.
The above call between LBJ and JC is from Holland's "The Assassination Tapes":
