  We present kinematic and photometric evidence for an accretion event in the
halo of the cD galaxy M87 in the last Gyr. Using velocities for ~300 planetary
nebulas (PNs) in the M87 halo, we identify a chevron-like substructure in the
PN phase-space. We implement a probabilistic Gaussian mixture model to identify
the PNs that belong to the chevron. From analysis of deep V-band images of M87,
we find that the region with the highest density of PNs associated to the
chevron, is a crown-shaped substructure in the optical light. We assign a total
of N_(PN,sub)=54 to the substructure, which extends over ~50 kpc along the
major axis where we also observe radial variations of the ellipticity profile
and a colour gradient. The substructure has highest surface brightness in a
20kpc x 60kpc region around 70 kpc in radius. In this region, it causes an
increase in surface brightness by >60%. The accretion event is consistent with
a progenitor galaxy with a V-band luminosity of L=2.8\pm1.0 x 10^9 L_(sun,V), a
colour of (B-V)=0.76\pm0.05, and a stellar mass of M=6.4\pm2.3 x 10^9 M_sun.
The accretion of this progenitor galaxy has caused an important modification of
the outer halo of M87 in the last Gyr. By itself it is strong evidence that the
galaxy's cD halo is growing through the accretion of smaller galaxies as
predicted by hierarchical galaxy evolution models.
