Watching the news. TEPCO representitives say they will not comment on the cause of the explosion or the possibility of leaked radioactive material until they have collected more data; but at the same rate they refuse to confirm that there was an explosion, or which building was affected. TEPCO is only confirming that 4 workers were injured and have been taken to a hospital; and that "the sound of an explosion" was heard (at 15:36 local time) and shortly after one building was compromised. This despite video of what was clearly a blast, and detailed photography which clearly reveals which specific building was compromised.
There was a comment from one news station suggesting that the explosion might have occured at the building which contains the turbines rather than the reactor, but TEPCO will not confirm this (despite now having several hours to conduct an investigation). For reference, reactors are generally built deeper into the ground within their own buildings and not at surface level; however if a reactor containment vessel is already breached, the enclosing building is the last line of containing radioactive material from freely escaping into the atmosphere.
I've got a very bad impression that TEPCO is woefully unprepared for emergency response, and that there is no central leadership for handling the situation. They seem caught between following PM Kan for making decisions (who is not an expert on nuclear safety), and relying on international assistance [and international assistance is there to assist, not to provide leadership on managing the crisis].
In the meantime, evacuation has been increased to residents within 10km of both plants. Most residents within the 10km distance of Plant 1 had already evacuated following the original procedure. However, the original procedure only included residents within 3km of Plant 2; now that the radius has been increased, there are still many residents who haven't been able to evacuated. Helicopters are now being sent to assist in air evacuations.
There are reports that journalists trying to get to Fukushima to cover the situation are now being stopped by police as far away as 60km away from the plants.
Meanwhile, TEPCO representitives continue to insist that they don't know anything and therefore can't provide any concrete information...