  At the beginning of the reionization epoch, radiation sources produce
fluctuations in the redshifted 21-cm background. We show that different types
of sources (such as miniquasars, Pop II and III stars, supernovae, etc.)
produce distinct signatures in the 21-cm signal radial profiles and statistical
fluctuations, through which they can be identified. Further, we show that the
21-cm signal from X-ray emitting sources is much easier to observe than was
expected, due to a previously neglected pumping mechanism.
