Amino acids have been identified in carbonaceous chondrites, but their origin is yet unknown. Previous work has shown that a variety of amino acids can be formed via ultraviolet photolysis of interstellar ice analogs. Two possible mechanisms of formation of these amino acids have been proposed: a Strecker-type synthesis or a radical-radical mechanism. In this work, we have used isotopic labeling techniques to test the predictions made by each of these proposed mechanisms for the formation of the amino acids glycine and serine. We observe that amino acid formation occurs via multiple pathways, with potentially different mechanisms for glycine and serine. The major reaction paths do not match either of the two predicted mechanisms, although a modified radical-radical mechanism may account for our observations. The observation of multiple routes suggests that the formation of amino acids in interstellar ice analogs is not narrowly dependent on ice composition, but may occur under a variety of conditions that influence product distributions.