Preliminary analysis of the data and debris suggests there was a "large breach" in one of the rocket's cryogenically cold helium systems. The rupture occurred in the helium system of an oxygen fuel tank in the second stage--the upper part of the rocket that separates from the main booster after liftoff, and which fires up to carry the payload to higher altitudes.
Helium is typically used to pressurize the oxygen and kerosene rocket fuels. It's stored at very high pressures, says Paulo Lozano, an aerospace engineer at MIT, but exactly how a breach would lead to the explosion is hard to say at this point.
The cause of the breach remains unknown as well. The investigation continues.