"Jaczko did not say Wednesday how the information was obtained, but the NRC and U.S. Department of Energy both have experts on site at the Fukushima Dai-ichi complex of six reactors. He said the spent fuel pool of the complex's Unit 4 reactor has lost water."
He may be right, but if so it is probably speculation, same as everyone else. At the risk of engaging in a logical fallacy, I am also a little skeptical of what Gregory Jaczko says, because he is a long time nuclear critic (he admits it on his web page) and was Harry Reid's science advisor. Harry Reid killed the Yucca Mountain fuel repository project, which in the future will be the reason why used fuel is stored in pools at plant sites, which opens up the same questions about fuel storage pools here.