The Collaborative International Dictionary of English v.0.48:

Circumstantiate \Cir`cum*stan"ti*ate\, v. t. [imp. & p. p.
   Circumstantiated; p. pr. & vb. n. Circumstantiating.]
   [See Circumstantiating (?).]
   [1913 Webster]
   1. To place in particular circumstances; to invest with
      particular accidents or adjuncts. [R.]
      [1913 Webster]

            If the act were otherwise circumstantiated, it might
            will that freely which now it wills reluctantly.
                                                  --Bramhall.
      [1913 Webster]

   2. To prove or confirm by circumstances; to enter into
      details concerning.
      [1913 Webster]

            Neither will time permint to circumstantiate these
            particulars, which I have only touched in the
            general.                              --State Trials
                                                  (1661).
      [1913 Webster]

WordNet (r) 3.0 (2006):

circumstantiate
    v 1: give circumstantial evidence for

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "circumstantiate":
   adduce, affirm, anatomize, atomize, attest, authenticate, back,
   back up, bear out, bolster, buttress, certify, cite, confirm,
   corroborate, descend to particulars, detail, document,
   enter into detail, fortify, give full particulars, instance,
   itemize, particularize, probate, prove, ratify, reinforce, specify,
   spell out, strengthen, substantiate, support, sustain, undergird,
   uphold, validate, verify, warrant