  In this paper we give a criterion for pairs of isometries of a nonpositively
curved metric space to generate a free group. This criterion holds only in
singular spaces, for example in Euclidean buildings.
  The original motivation for our criterion was to prove that the four
dimensional Burau representation is faithful. Although we do not settle this
question, we do exhibit a related 2-parameter family of faithful
representations of the free group F_2.
