  We present a four-body semileptonic charm decay D^+ into K^-pi^+mu^+nu
analysis in the range of 0.65 GeV/c^2 < mkpi < 1.5 GeV/c^2. We observe a low
mass scalar contribution of 5.30 +- 0.74 + 0.99 - 0.96 % with respect to the
total D^+ into K^-pi^+mu^+nu decay, compatible with the phase shift found by
the LASS elastic scattering experiment. For the K*(892) resonance, we obtain a
mass of 895.41 +- 0.32 + 0.35 - 0.43 MeV/c^2, a width of 47.79 +- 0.86 + 1.32 -
1.06 MeV/c^2, and a Blatt-Weisskopf damping factor parameter of 3.96 +- 0.54 +
1.31 - 0.90 GeV^(-1). We also report 90% CL upper limits of 4% and 0.64% for
the branching ratios Gamma(D^+ into K*(1680)mu^+nu)/Gamma(D^+ into
K^-pi^+mu^+nu) and Gamma(D^+ into K*_0(1430)mu^+nu)/Gamma(D^+ into
K^-pi^+mu^+nu), respectively.
