apprehender

from WordNet (r) 3.0 (2006)

apprehender
    n 1: a person who knows or apprehends [syn: {knower},
         {apprehender}]
    2: a person who seizes or arrests (especially a person who
       seizes or arrests in the name of justice)

from The Collaborative International Dictionary of English v.0.48

Apprehender \Ap`pre*hend"er\, n.
   One who apprehends.
   [1913 Webster]