  We describe the formation of a seed massive black hole (MBH) inside a
supermassive star (SMS) in a distant galactic nucleus. The short-lived SMS is
naturally formed due to collision destructions of normal stars in the evolving
galactic nucleus. The neutron stars (NSs) and stellar-mass black holes form a
compact self-gravitating subsystem deep inside a SMS. This subsystem is
short-lived in comparison with a host SMS and collapses finally into the MBH.
Just before gravitational collapse of compact subsystem the frequent NS
collisions are accompanied by the generation of numerous ultra-relativistic
fireballs. A combined ram pressure of multiple coexisting fireballs produces a
quasi-stationary rarefied cavity in the central part of SMS. The protons are
accelerated in the fireballs and by relativistic shocks in the cavity. All
secondary particles, produced in collisions, except the high-energy neutrinos
are absorbed in the SMS interiors. An estimated high-energy neutrino signal
from this hidden source can be detected by the neutrino telescope with an
effective area 1 km^2 providing the evidence for MBH formation in a distant
galactic nucleus. A corresponding lifetime of this high-energy hidden neutrino
source is ~0.1-1 yr.
