  We present Keck optical spectra of SN 2002cx, the most peculiar known Type Ia
supernova (SN Ia), taken 227 and 277 days past maximum light. Astonishingly,
the spectra are not dominated by the forbidden emission lines of iron that are
a hallmark of thermonuclear supernovae in the nebular phase. Instead, we
identify numerous P-Cygni profiles of Fe II at very low expansion velocities of
about 700 km/s, which are without precedent in SNe Ia. We also report the
tentative identification of low-velocity O I in these spectra, suggesting the
presence of unburned material near the center of the exploding white dwarf. SN
2002cx is the prototype of a new subclass of SNe Ia, with spectral
characteristics that may be consistent with recent pure deflagration models of
Chandrasekhar-mass thermonuclear supernovae. These are distinct from the
majority of SNe Ia, for which an alternative explosion mechanism, such as a
delayed detonation, may be required.
