  We formulate the general construction for singular vectors in Verma modules
of the affine sl(2|1) superalgebra. We then construct sl(2|1) representations
out of the fields of the non-critical N=2 string. This allows us to extend
naturally to sl(2|1) several crucial properties of the N=2 superconformal
algebra, first of all the construction of extremal states (an analogue of
different pictures for non-free fermions) and the spectral flow transform
(which then affects the Liouville sector). We further evaluate the affine
sl(2|1) singular vectors in the realization of sl(2|1) provided by the N=2
string. We establish that, with a notable exception, the respective singular
vectors are in a 2:1 correspondence, namely two different sl(2|1) singular
vectors evaluate as an N=2 superconformal singular vector (however, those
singular vectors that are labelled by a pair of positive integers get these
integers transposed under the reduction). We also analyse the `exceptional'
cases, which amount to a series of sl(2|1) singular vectors, labelled by r>=1,
which do not have an N=2 counterpart, and discuss the mechanism by which the
multiplicity of singular vectors becomes equal to two at certain points in the
weight spaces of both algebras.
