  We report the first results on a direct search for a new 16.7 MeV boson (X)
which could explain the anomalous excess of e+e- pairs observed in the excited
Be-8 nucleus decays. Due to its coupling to electrons, the X could be produced
in the bremsstrahlung reaction e- Z -> e- Z X by a 100 GeV e- beam incident on
an active target in the NA64 experiment at the CERN SPS and observed through
the subsequent decay into an e+e- pair. With 5.4\times 10^{10} electrons on
target, no evidence for such decays was found, allowing to set first limits on
the X-e^- coupling in the range 1.3\times 10^{-4} < \epsilon_e < 4.2\times
10^{-4} excluding part of the allowed parameter space. We also set new bounds
on the mixing strength of photons with dark photons (A') from non-observation
of the decay A'->e+e- of the bremsstrahlung A' with a mass <~ 23 MeV.
