  An anomaly in time distribution of neutrinos from the ISIS pulsed beam stop
source observed by the KARMEN collaboration is discussed. We show that the
anomaly can be interpreted as a superposition of two exponentials, both having
time constants consistent with the $\mu^{+}$ lifetime of 2.2 $\mu s$. % and,
therefore, It is assumed that they both originate from muon decays at rest.
  One of them describes the time distribution of the prompt neutrino events,
while the other describes the time distribution of events from delayed decays
of slowly moving ($\beta \simeq 0.02$) particles in the KARMEN calorimeter.
  We propose here that these particles are produced in exotic decays of
positive muons $\mu^{+}\to e^{+} + X$, resulting in the second exponential time
distribution shifted by the time of flight with respect to the time
distribution of neutrino events. This model gives an aceptable fit to the
KARMEN data if $X$ has a mass of 103.9 MeV.
  The possible decay modes of this new massive neutral particle are discussed.
This hypothesis can be experimentally tested in the near future by studying the
low energy part of the $e^{+}$ spectrum in the $\mu^{+}$ decays.
