  VLT spectroscopic observations of Ly alpha--excess objects in the field of
the clumpy radio galaxy 1138-262 at z=2.16 have led to the discovery of 14
galaxies and one QSO at approximately the same distance as the radio galaxy.
All galaxies have redshifts in the range 2.16 \pm 0.02, centered around the
redshift of the radio galaxy, and are within a projected physical distance of
1.5 Mpc from it. The velocity distribution suggests that there are two galaxy
subgroups having velocity dispersions of ~500 km/s and ~300 km/s and a relative
velocity of 1800 km/s. If these are virialized structures, the estimated
dynamical masses for the subgroups are ~9 and ~3x10E13 solar masses
respectively, implying a total mass for the structure of more than 10E14 solar
masses. The new observations, together with previous results, suggest that the
structure of galaxies around 1138-262 is likely to be a forming cluster.
