  Analyzing archival data of the INTEGRAL observatory we discovered and
localized a cosmic gamma-ray burst detected within fields of view of the
IBIS/ISGRI and SPI telescopes on April 28, 2006. The burst has not been
revealed by the INTEGRAL Burst Alert System (IBAS), so the information on its
coordinates has not been distributed in time and search for the afterglow has
not been carried out. The burst has been also detected by the KONUS/WIND and
RHESSI spacecrafts. Its fluence was 2.3x10^{-6} erg/cm^2 in the 20-200 keV
energy band, the maximum flux was 3.6x10^{-7} erg/cm^2/s (3.9 phot/cm^2/s). The
burst had a complicated multi-peak profile and was outstanding of the typical
bursts by increasing its spectral hardness with time. The emission spectrum
near the flux maximum was characterized by the photon index alpha ~ -1.5 and
the peak energy Ep ~ 95 keV. The burst lasted ~12 s, then we observed its
afterglow at energies 15-45 keV decaying according to a power law with the
index ~ -4.5. The spectral hardness decreased substantially during the
afterglow.
