No further information such as:
What the animal was;
Whether the animal was singed or burned;
How the animal died; or
Whether the wiring had been damaged (by the animal).
What we do know is the animal is about 15 centimeters long, or 25 centimeters including the tail.
Asahi's "Hodo Station" news apparently made a decision on its own that the animal was burned to death, and reported as such. But from the TEPCO's afternoon press conference on March 20, 2013, there was no reference to how the animal, which does look like a rat, died.
From TEPCO's Photos and Videos on March 20, 2013:
3/4 M/C (A) Switchboard box on what looks like a trailer bed:
Workers inspecting 3/4 M/C (A):
The bottom compartment looks fine:
But above, the terminals look burned:
And the compartment wall is sooty:
And here's the animal, at the bottom of the compartment (photo taken from above):
From my personal experience, there is no way to prevent rodents from entering any enclosure and build nests. You have to be very vigilant and monitor regularly, and remove the first bits of nesting materials or shit as soon as you see them. If this switchboard enclosure has been there on the the trailer since March 18, 2011 without regular inspection, I would be surprised if there were no rat's nest.
"TEPCO" stands for Tokyo Electric Power Company. It should know very well how these critters behave.
On second thoughts, such knowledge was probably held by TEPCO's subcontractors, not by TEPCO.