  The LIGO/Virgo Collaboration (LVC) detected on 2017 January 4, a significant
gravitational-wave (GW) event (now named GW170104). We report in this Letter
the main results obtained from the analysis of hard X-ray and gamma-ray data of
the AGILE mission that repeatedly observed the GW170104 localization region
(LR). At the LVC detection time $T_0$ AGILE observed about 36% of the LR. The
gamma-ray imaging detector did not reveal any significant emission in the
energy range 50 MeV--30 GeV. Furthermore, no significant gamma-ray transients
were detected in the LR that was repeatedly exposed over timescales of minutes,
hours, and days. We also searched for transient emission using data near $T_0$
of the omnidirectional detector MCAL operating in the energy band 0.4--100 MeV.
A refined analysis of MCAL data shows the existence of a weak event (that we
call "E2") with a signal-to-noise ratio of $4.4\,\sigma$ lasting about 32 ms
and occurring $0.46\,\pm\,0.05 \,\rm s$ before $T_0$. A study of the MCAL
background and of the false-alarm rate of E2 leads to the determinination of a
post-trial significance between $2.4\,\sigma$ and $2.7\,\sigma$ for a temporal
coincidence with GW170104. We note that E2 has characteristics similar to those
detected from the weak precursor of the short GRB 090510. The candidate event
E2 is worth consideration for simultaneous detection by other satellites. If
associated with GW170104, it shows emission in the MeV band of a short burst
preceding the final coalescence by 0.46 sec and involving $\sim 10^{-7}$ of the
total rest mass energy of the system.
