  We present deep Hubble Space Telescope Wide Field Camera 3 slitless
spectroscopic observations of the distant cluster Cl J1449+0856. These cover a
single pointing with 18 orbits of G141 spectroscopy and F140W imaging, allowing
us to derive secure redshifts down to m_140~25.5 AB and 3sigma line fluxes of
5*10^(-18) erg/s/cm^2. In particular, we were able to spectroscopically confirm
12 early-type galaxies in the field up to z~3, 6 of which in the cluster core,
which represents the first direct spectroscopic confirmation of passive
galaxies in a z=2 cluster environment. With 140 redshifts in a ~6 arcmin^2
field, we can trace the spatial and redshift galaxy distribution in the cluster
core and background field. We find two strong peaks at z=2.00 and z=2.07, where
only one was seen in our previously published ground-based data. Thanks to the
spectroscopic confirmation of the cluster ETGs, we can now re-evaluate the
redshift of Cl J1449+0856 at z=2.00, rather than z=2.07, with the background
overdensity being revealed to be sparse and "sheet"-like. This presents an
interesting case of chance alignment of two close yet unrelated structures,
each one preferentially selected by different observing strategies. With 6
quiescent or early-type spectroscopic members and 20 star-forming ones, Cl
J1449+0856 is now reliably confirmed to be at z=2.00. The identified members
can now allow for a detailed study of galaxy properties in the densest
environment at z=2.
