The FBI needs access to a lot of information in order to support the Federal charges that were just announced today. This Loughner guy has been charged with various charges related to murder and attempted murder of Federal officials, and there may be some difficulty getting a conviction in the case if Loughner's defense team can successfully claim that the shootings were completely random and he had no idea who his victims were.
In order to bolster their case, the Feds need to present sufficient evidence (in the form of written documents, e-mails, etc.) that Loughner targeted his victims (especially the Congresswoman) because of the Federal positions they held.
I don’t at all keep up with these things, so maybe you can enlighten me: are there different sets of laws for the murder or attempted murder of Federal officials/employees than there are for us commoners? I had no idea their lives counted more than or differently than ours.