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
8
Be
∗
nucleus decays. Because of 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 Super Proton Synchrotron and observed through the subsequent decay into a
e
+
e
−
pair. With
5.4
×
10
10
electrons on target, no evidence for such decays was found, allowing us to set first limits on the
X
−
e
−
coupling in the range
1.3
×
10
−
4
≲
ε
e
≲
4.2
×
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 nonobservation of the decay
A
′
→
e
+
e
−
of the bremsstrahlung
A
′
with a mass
≲
23
Me
