A search for long-lived neutral particles (N-0's) with masses above 2.2 GeV
/c(2) that decay into at least one muon has been performed using an instrum
ented decay channel at the NuTeV experiment at Fermilab. Data were examined
for particles decaying into the final states mu mu, mue, and mu pi. Three
mu mu events were observed over an expected standard model background of 0.
069 +/- 0.010 events; no events were observed in the other modes.