  Using data collected by the high energy photoproduction experiment FOCUS at
Fermilab we performed a Dalitz plot analysis of the Cabibbo favored decay D+ to
K-pi+ pi+. This study uses 53653 Dalitz-plot events with a signal fraction of ~
97%, and represents the highest statistics, most complete Dalitz plot analysis
for this channel. Results are presented and discussed using two different
formalisms. The first is a simple sum of Breit--Wigner functions with freely
fitted masses and widths. It is the model traditionally adopted and serves as
comparison with the already published analyses. The second uses a K-matrix
approach for the dominant S-wave, in which the parameters are fixed by first
fitting Kpi scattering data and continued to threshold by Chiral Perturbation
Theory. We show that the Dalitz plot distribution for this decay is consistent
with the assumption of two body dominance of the final state interactions and
the description of these interactions is in agreement with other data on the
Kpi final state.
