Source : Webster's Revised Unabridged Dictionary (1913)

Apprehender \Ap`pre*hend"er\, n.
   One who apprehends.

Source : WordNet®

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