  Based on the ACV approach to transplanckian energies, the reduced-action
model for the gravitational S-matrix predicts a critical impact parameter b_c ~
R = 2 G sqrt{s} such that S-matrix unitarity is satisfied in the perturbative
region b > b_c, while it is exponentially suppressed with respect to s in the
region b < b_c that we think corresponds to gravitational collapse. Here we
definitely confirm this statement by a detailed analysis of both the critical
region b ~ b_c and of further possible contributions due to quantum transitions
for b < b_c. We point out, however, that the subcritical unitarity suppression
is basically due to the boundary condition which insures that the solutions of
the model be ultraviolet-safe. As an alternative, relaxing such condition leads
to solutions which carry short-distance singularities presumably regularized by
the string. We suggest that through such solutions - depending on the detailed
dynamics at the string scale - the lost probability may be recovered.
