11:00 News showed a VERY clear still photo of a trim-tab coming off the plane just before the pilot lost full control.
The trim-tab is not that large a part, it is part of the horizontal stabilizer.
Apparently, losing it made this plane uncontrollable.
So, it seems fairly clear that this was a mechanical failure at speed.
No doubt the NTSB will have more to say, in several months.
http://www.youtube.com/watch?v=ndd9PVDM3jU Title: Plane Crash at Reno Air Race 2011 (Angle 2)