  The accretion of matter onto a massive black hole is believed to feed the
relativistic plasma jets found in many active galactic nuclei (AGN). Although
some AGN accelerate particles to energies exceeding 10^12 electron Volts (eV)
and are bright sources of very-high-energy (VHE) gamma-ray emission, it is not
yet known where the VHE emission originates. Here we report on radio and VHE
observations of the radio galaxy M87, revealing a period of extremely strong
VHE gamma-ray flares accompanied by a strong increase of the radio flux from
its nucleus. These results imply that charged particles are accelerated to very
high energies in the immediate vicinity of the black hole.
