11:00 News channel 8 showed a VERY clear still photo of a trim-tab coming off the plane’s tail 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 racing speed.
No doubt the NTSB will have more to say, in several months.