  The physical conditions giving rise to high escape fractions of ionizing
radiation (LyC $f_{\rm{esc}}$) in star-forming galaxies - most likely
protagonists of cosmic reionization - are not yet fully understood. Using the
VLT/MUSE observations of ~1400 Ly$\alpha$ emitters at 2.9 < z < 6.7, we compare
stacked rest-frame UV spectra of candidates for LyC leakers and non-leakers
selected based on their Ly$\alpha$ profiles. We find that the stacks of
potential LyC leakers, i.e. galaxies with narrow, symmetric Ly$\alpha$ profiles
with small peak separation, generally show (i) strong nebular OIII]1666,
[SiIII]1883, and [CIII]1907+CIII]1909 emission, indicating a high-ionization
state of the interstellar medium (ISM); (ii) high equivalent widths of HeII1640
(~1-3 A), suggesting the presence of hard ionizing radiation fields; (iii)
SiII*1533 emission, revealing substantial amounts of neutral hydrogen off the
line of sight; (iv) high CIV1548,1550 to [CIII]1907+CIII]1909 ratios (CIV/CIII]
> 0.75), signalling the presence of low column density channels in the ISM. In
contrast, the stacks with broad, asymmetric Ly$\alpha$ profiles with large peak
separation show weak nebular emission lines, low HeII1640 equivalent widths (<1
A), and low CIV/CIII] (<0.25), implying low-ionization states and high-neutral
hydrogen column densities. Our results suggest that CIV/CIII] might be
sensitive to the physical conditions that govern LyC photon escape, providing a
promising tool for identification of ionizing sources among star-forming
galaxies in the epoch of reionization.
