  $Ni Cl_2$-$4SC(NH_2)_2$ (known as DTN) is a spin-1 material with a strong
single-ion anisotropy that is regarded as a new candidate for Bose-Einstein
condensation (BEC) of spin degrees of freedom. We present a systematic study of
the low-energy excitation spectrum of DTN in the field-induced magnetically
ordered phase by means of high-field electron spin resonance measurements at
temperatures down to 0.45 K. We argue that two gapped modes observed in the
experiment can be consistently interpreted within a four-sublattice
antiferromagnet model with a finite interaction between two tetragonal
subsystems and unbroken axial symmetry. The latter is crucial for the
interpretation of the field-induced ordering in DTN in terms of BEC.
