  We use the recent detection of He II absorption at $z=2.2-2.6$ in the line of
sight of the quasar HS1700+64 to put bounds on the sources of ionization. We
find that given the uncertainty in $\tauhgp$ and the model of absorption in the
intergalactic medium (IGM), a wide range of possible sources of ionization is
still allowed by the observations. We show that a significant contribution from
star forming galaxies is allowed and is consistent with the proximity effect.
In the case of photoionization by the UV background radiation, the contribution
of star forming galaxies to the intensity at $912 > {\AA}$ can be within a
range of $\sim 1 - 15$ times that of the quasars. We also investigate the case
of a collisionally ionized intergalactic medium. We show that although
collisional ionization can be a dominant source of ionization at $z \sim 2.4$
(taking into account the upper limit on Compton y-parameter of the microwave
background radiation), the thermal state of the IGM cannot yet be determined.
We also consider Sciama's model of radiatively decaying neutrinos and show that
the model of decaying neutrino is consistent with the observations.
