What Was The Role of David Herold

[Gene C]

And recall, lastly, that Robert Lincoln said, in later years, that he had positive proof that a member of his father's cabinet had been guilty of treason, a statement that ties in well with Jacob Thompson's statement that he would not write memoirs because to do so would positively ruin a man who was still active in national politics.

Anyone have any ideas of who this could be?
(Treason does not necessarily mean involvement in the assassination)