  Regularity of the horizon radius $r_g$ of a collapsing body constrains a
limiting form of a spherically-symmetric energy-momentum tensor near it. Its
non-zero limit belongs to one of four classes that are distinguished only by
two signs. As a result, close to $r_g$ the geometry can always be described by
either an ingoing or outgoing Vaidya metric with increasing or decreasing mass.
If according to a distant outside observer the trapped regions form in finite
time, then the Einstein equations imply violation of the null energy condition.
In this case the horizon radius and its rate of change determine the metric in
its vicinity, and the hypersurface $r = r_g(t)$ is timelike during both the
expansion and contraction of the trapped region. We present the implications of
these results for the firewall paradox and discuss arguments that the required
violation of the null energy condition is incompatible with the standard
analysis of black hole evaporation.
