  We analyze scenarios of particle collisions in the metric of a nonextremal
black hole that can potentially lead to ultrahigh energy $E_{c.m.}$ in their
centre of mass frame. Particle 1 comes from infinity to the black hole horizon
while particle 2 emerges from a white hole region. It is shown that unbounded
$E_{c.m.}$ $\ $require that particle 2 pass close to the bifurcation point. The
analogy with collisions inside the horizon is discussed.
