  We investigate the afterglow of GRB 140713A, a gamma-ray burst (GRB) that was
detected and relatively well-sampled at X-ray and radio wavelengths, but was
not present at optical and near-infrared wavelengths, despite searches to deep
limits. We present the emission spectrum of the likely host galaxy at $z =
0.935$ ruling out a high-redshift explanation for the absence of the optical
flux detection. Modelling the GRB multi-wavelength afterglow using the
radiative transfer hydrodynamics code BOXFIT provides constraints on physical
parameters of the GRB jet and its environment, for instance a relatively wide
jet opening angle and an electron energy distribution slope $p$ below 2. Most
importantly, the model predicts an optical flux about two orders of magnitude
above the observed limits. We calculated that the required host extinction to
explain the observed limits in the $r$, $i$ and $z$ bands was $A^{\rm host}_{V}
> 3.2$ mag, equivalent to $E(B-V)^{\rm host} > 1.0$ mag. From the X-ray
absorption we derive that the GRB host extinction is $A^{\rm host}_{\rm V} =
11.6^{+7.5}_{-5.3}$ mag, equivalent to $E(B-V)^{\rm host} = 3.7^{+2.4}_{-1.7}$
mag, which is consistent with the extinction required from our BOXFIT derived
fluxes. We conclude that the origin of the optical darkness is a high level of
extinction in the line of sight to the GRB, most likely within the GRB host
galaxy.
