From an accredated site (http://www2.slac.stanford.edu/vvc/theory/relativity.html):
"Notice that this equation tells you that for any particle with a non-zero mass, the momentum gets larger and larger as the speed gets closer to the speed of light (Cher's cliffnote: MOMENTUM...NOT mass). Such a particle would have infinite momentum if it could reach the speed of light. Since it would take an infinite amount of force (or a finite force acting over an infinite amount of time) to accelerate a particle to infinite momentum, we are forced to conclude that a massive particle always travels at speeds less than the speed of light."
Furthermore:
"Another interesting fact about the expression that relates E and p above (E2 = m2c4 + p2c2), is that it is also true for the case where a particle has no mass (m=0). In this case, the particle always travels at a speed c, the speed of light. You can regard this equation as a definition of momentum for such a mass-less particle. Photons have kinetic energy and momentum, but no mass!"
To put this in terms that aren't so complicated, it clearly states that anything that doesn't have mass is constantly traveling at the speed of light, MEANING in order for something to be traveling that fast it has to have a decrease in mass, NOT increase. Obviously, light particles (photons) have no mass, meaning that mass must decrease in order to increase energy and become light.