  We present an analysis of the longest timescale microlensing events
discovered by the MACHO Collaboration during a 7 year survey of the Galactic
bulge. We find 6 events that exhibit very strong microlensing parallax signals
due, in part, to accurate photometric data from the GMAN and MPS
collaborations. The microlensing parallax fit parameters are used in a
likelihood analysis, which is able to estimate the distance and masses of the
lens objects based upon a standard model of the Galactic velocity distribution.
This analysis indicates that the most likely masses of 5 of the 6 lenses are >
1 Msun, which suggests that a substantial fraction of the Galactic lenses may
be massive stellar remnants. This could explain the observed excess of long
timescale microlensing events. The lenses for events MACHO-96-BLG-5 and
MACHO-98-BLG-6 are the most massive, with mass estimates of M/Msun = 6 +10/-3
and M/Msun = 6 +7/-3, respectively. The observed upper limits on the absolute
brightness of main sequence stars for these lenses are < 1 Lsun, so both lenses
are black hole candidates. The black hole interpretation is also favored by a
likelihood analysis with a Bayesian prior using a conventional model for the
lens mass function. We consider the possibility that the source stars for some
of these 6 events may lie in the foreground or background of the Galactic
bulge, but we find that this is unlikely. Future HST observations of these
events can either confirm the black hole lens hypothesis or detect the lens
stars and provide a direct measurement of their masses. Future observations of
similar events by SIM or the Keck or VLTI interferometers will allow direct
measurements of the lens masses for stellar remnant lenses as well.
