The Collaborative International Dictionary of English v.0.48:

mitigating \mitigating\ adj.
   serving to reduce blame; -- of situations; as, mitigating
   factors; mitigating circumstances. Opposite of aggravating.
   [Narrower terms: exculpatory]

   Syn: extenuating.
        [WordNet 1.5 +PJC]

The Collaborative International Dictionary of English v.0.48:

Exculpatory \Ex*cul"pa*to*ry\
   Clearing, or tending to clear, from alleged fault or guilt;
   excusing. "An exculpatory letter." --Johnson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exculpatory
    adj 1: clearing of guilt or blame [ant: inculpative,
           inculpatory]