  Long gamma-ray bursts mark the death of massive stars, as revealed by their
association with energetic broad-lined stripped-envelope supernovae. The
scarcity of nearby events and the brightness of the GRB afterglow, dominating
the first days of emission, have so far prevented the study of the very early
stages of the GRB-SN evolution. Here we present detailed, multi-epoch
spectroscopic observations of SN 2017iuk, associated with GRB 171205A which
display features at extremely high expansion velocities of $\sim$ 100,000 km
s$^{-1}$ within the first day after the burst. These high-velocity components
are characterized by chemical abundances different from those observed in the
ejecta of SN 2017iuk at later times. Using spectral synthesis models developed
for the SN 2017iuk, we explain these early features as originating not from the
supernova ejecta, but from a hot cocoon generated by the energy injection of a
mildly-relativistic GRB jet expanding into the medium surrounding the
progenitor star. This cocoon becomes rapidly transparent and is outshone by the
supernova emission which starts dominating three days after the burst. These
results proves that the jet plays an important role not only in powering the
GRB event but also its associated supernova.
