  The complex phenomenology shown by the UV-bright, variable spike first
detected with the Hubble Space Telescope (HST) at the center of the otherwise
normal galaxy NGC 4552 is further investigated with both HST imaging (FOC) and
spectroscopy (FOS). HST/FOC images taken in 1991, 1993, and 1996 in the near UV
have been analyzed in a homogeneous fashion, showing that the central spike has
brightened by a factor ~4.5 between 1991 and 1993, and has decreased its
luminosity by a factor ~2.0 between 1993 and 1996. FOS spectroscopy extending
from the near UV to the red side of the optical spectrum reveals a strong UV
continuum over the spectrum of the underlying galaxy, along with several
emission lines in both the UV and the optical ranges. In spite of the low
luminosity of the UV continuum of the spike (~3*10^5 Lsolar), the spike is
definitely placed among AGNs by current diagnostics based on the emission line
intensity ratios, being just on the borderline between Seyferts and LINERs.
Line profiles are very broad, and both permitted and forbidden lines are best
modelled with a combination of broad and narrow components, with FWHM of ~3000
km s^-1 and ~700 km s^-1, respectively. This evidence argues for the variable
central spike being produced by a modest accretion event onto a central massive
black hole (BH), with the accreted material having possibly being stripped from
a a star in a close fly by with the BH. The 1996 broad Halpha luminosity of
this mini-AGN is ~5.6*10^37 erg s^-1, about a factor of two less than that of
the nucleus of NGC 4395, heretofore considered to be the faintest known AGN.
