  We present the results of broadband near-infrared spectroscopic observations
of the recently discovered mysterious stellar objects in the young supernova
remnant G54.1+0.3. These objects, which show significant mid-infrared excess
emission, are embedded in a diffuse loop structure of ~1 arcmin in radius.
Their near-infrared spectra reveal characteristics of late O- or early B-type
stars with numerous H and He I absorption lines, and we classify their spectral
types to be between O9 and B2 based on an empirical relation derived here
between the equivalent widths of the H lines and stellar photospheric
temperatures. The spectral types, combined with the results of spectral energy
distribution fits, constrain the distance to the objects to be 6.0 +/- 0.4 kpc.
The photometric spectral types of the objects are consistent with those from
the spectroscopic analyses, and the extinction distributions indicate a local
enhancement of matter in the western part of the loop. If these objects
originate via triggered formation by the progenitor star of G54.1+0.3, then
their formations likely began during the later evolutionary stages of the
progenitor, although a rather earlier formation may still be possible. If the
objects and the progenitor belong to the same cluster of stars, then our
results constrain the progenitor mass of G54.1+0.3 to be between 18 and ~35
M_sun and suggest that G54.1+0.3 was either a Type IIP supernova or, with a
relatively lower possibility, Type Ib/c from a binary system.
