  Multiple theories have been proposed to describe the formation of black hole
seeds in the early Universe and to explain the emergence of very massive black
holes observed in the first billion years after Big Bang. Models consider
different seeding and accretion scenarios, which require the detection and
characterisation of black holes in the first few hundred million years after
Big Bang to be validated. Here we present an extensive analysis of the
JWST-NIRSpec spectrum of GN-z11, an exceptionally luminous galaxy at z=10.6,
revealing the detection of the [NeIV]2423 and CII*1335 transitions (typical of
Active Galactic Nuclei, AGN), as well as semi-forbidden nebular lines tracing
gas densities higher than 10^9 cm-3, typical of the Broad Line Region of AGN.
These spectral features indicate that GN-z11 hosts an accreting black hole. The
spectrum also reveals a deep and blueshifted CIV1549 absorption trough, tracing
an outflow with velocity 800-1000 km/s, likely driven by the AGN. Assuming
local virial relations, we derive a black hole mass of log(M_BH/Msun) = 6.2 +-
0.3, accreting at about 5 times the Eddington rate. These properties are
consistent with both heavy seeds scenarios, or scenarios envisaging
intermediate/light seeds experiencing episodic super-Eddington phases. Our
finding naturally explains the high luminosity of GN-z11 and can also provide
an explanation for its exceptionally high nitrogen abundance.
