  A search for long-lived neutral particles (N^0) which decay into at least one
muon has been performed using an instrumented decay channel at the E815 (NuTeV)
experiment at Fermilab. The decay channel was composed of helium bags
interspersed with drift chambers, and was used in conjunction with the NuTeV
neutrino detector to search for N^0 decays. The data were examined for
particles decaying into the muonic final states mu mu, mu e, and mu pi. Three
mu mu events were observed over an expected background of 0.040 +/- 0.009
events; no events were observed in the other modes. Although the observed
events share some characteristics with neutrino interactions, the observed rate
is a factor of 75 greater than expected. No Standard Model process appears to
be consistent with this observation.
