The Collaborative International Dictionary of English v.0.48:

Mysteriously \Mys*te"ri*ous*ly\, adv.
   In a mysterious manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mysteriously
    adv 1: in a cryptic manner; "we will meet again," he said
           cryptically [syn: cryptically, enigmatically,
           mysteriously]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "mysteriously":
   awe-inspiringly, awesomely, awfully, awingly, bizarrely, eerily,
   enigmatically, fancifully, fantastically, grotesquely,
   inexplicably, monstrously, mystically, numinously, outlandishly,
   peculiarly, quaintly, queerly, singularly, strangely,
   supernaturally, unaccountably, uncannily, undefinably,
   unexplainably, weirdly