  Gamma-ray emission in the MeV-GeV range from explosive cosmic events is of
invaluable relevance to understanding physical processes related to the
formation of neutron stars and black holes. Here we report on the detection by
the AGILE satellite in the MeV-GeV energy range of the remarkable long-duration
gamma-ray burst GRB 221009A. The AGILE onboard detectors have good exposure to
GRB 221009A during its initial crucial phases. Hard X-ray/MeV emission in the
prompt phase lasted hundreds of seconds, with the brightest radiation being
emitted between 200 and 300 seconds after the initial trigger. Very intense GeV
gamma-ray emission is detected by AGILE in the prompt and early afterglow phase
up to 10,000 seconds. Time-resolved spectral analysis shows time-variable
MeV-peaked emission simultaneous with intense power-law GeV radiation that
persists in the afterglow phase. The coexistence during the prompt phase of
very intense MeV emission together with highly nonthermal and hardening GeV
radiation is a remarkable feature of GRB 221009A. During the prompt phase, the
event shows spectrally different MeV and GeV emissions that are most likely
generated by physical mechanisms occurring in different locations. AGILE
observations provide crucial flux and spectral gamma-ray information regarding
the early phases of GRB 221009A during which emission in the TeV range was
reported.
