As for Calibri, even Microsoft has moved on. They designed a successor font family for even better readability, and that happened before the State Department changed to Calibri. It also has serif variants.
The Warren Commission report was set in Century Schoolbook, the Supreme Court's typeface of choice. The appendices are photo reproductions of originals produced on typewriters, so they are in something monospaced.
You can see for yourself. They certainly would not have used Times New Roman.
But I suppose interoffice memoranda are meant to be skimmed, not read, so TNR or Calibri are both fine.
What I want to know is what they were doing before 2004 when they adopted Times New Roman. Courier? Gothic?
Calibri is a nice screen-reading typeface. It is likely that "upstairs" reads on a screen, but I wouldn't bet that much on it.
Times New Roman was designed to shove as many letters into a multi-column newspaper page as possible. It's atrocious for anything else, and should be confined to whence it came, print newspapers. It's only the default because it is the default. Good riddance.