  In this work we present the interpretation of the energy spectrum and mass
composition data as measured by the Pierre Auger Collaboration above $6 \times
10^{17}$ eV. We use an astrophysical model with two extragalactic source
populations to model the hardening of the cosmic-ray flux at around $5\times
10^{18}$ eV (the so-called "ankle" feature) as a transition between these two
components. We find our data to be well reproduced if sources above the ankle
emit a mixed composition with a hard spectrum and a low rigidity cutoff. The
component below the ankle is required to have a very soft spectrum and a mix of
protons and intermediate-mass nuclei. The origin of this intermediate-mass
component is not well constrained and it could originate from either Galactic
or extragalactic sources. To the aim of evaluating our capability to constrain
astrophysical models, we discuss the impact on the fit results of the main
experimental systematic uncertainties and of the assumptions about quantities
affecting the air shower development as well as the propagation and redshift
distribution of injected ultra-high-energy cosmic rays (UHECRs).
