  In a recent study of a search for enhancements from the galactic center with
muons at sea level using the TUPI muon telescope, we have found several ground
level enhancements (GLEs) as very sharp peaks above the count rate background.
This paper reports a consistent analysis of two GLEs observed in December 2003
and detected after an up-grade of the data acquisition system, which includes a
noise filter and which allows us to verify that the GLEs are not mere
background fluctuations. The main target of this study is a search for the
origin of the GLEs. The results show that one of them has a strong correlation
with a solar flare, while the other has an unknown origin, because there is
neither a satellite report of a solar flare, nor prompt X-ray emission, and nor
a excess of nuclei during the raster scan where the GLE was observed. Even so,
two possibilities are analyzed: the solar flare hypothesis and the gamma ray
burst (GRB) hypothesis. We show, by using the FLUKA Monte Carlo results for
photo-production, that under certain conditions there is the possibility of an
enhancement of muons at sea level from GeV GRBs.
