Unfortunately, the car didn't have a data logging system installed which might have helped to isolate the cause. No one was injured and the battery pack sustained some damage but was apparently intact and functional - implying that it was not the cause of the fire. What this points out is that before plug-in hybrids and EVs are brought to market a lot of engineering and validation testing needs to be done to ensure that all systems in the car are safe, durable and properly integrated. This is actually the part of vehicle development that often takes the most time. It's not just the batteries that have to work, but all the bits and pieces around it. That's why it's taking almost four years from concept to production for the Volt and why Toyota is in no rush to bring the PHEV Prius to market.
[Source: Cooperative Research Network, thanks to the un-named reader for the tip!]