We report a study of the invariant mass distribution of jet
pairs produced in association with a W boson using data collected with the
CDF detector which correspond to an integrated luminosity of 4.3 fb-1.
The observed distribution has an excess in the 120-160 GeV/c2 mass
range which is not described by current theoretical predictions within
the statistical and systematic uncertainties. In this letter we report
studies of the properties of this excess.