On Friday, September 28, 2018 at 9:03:54 AM UTC-7, wrote:
What pieces of the wreckage were retrieved?...

The NTSB preliminary report contains a picture of the wreckage. Its condition makes clear that only the metal parts, and only some of those, will provide any meaningful clues.

Maybe a tool left under the seat, which eventually jammed the elevator?

Could be that. Given the facts of the last known location and direction of the glider, and the fact that it had enough kinetic energy to perform the maneuvers described, my speculation would be towards a progressive elevator system interference that first made it difficult to reduce speed (hence enough airspeed to perform two successive loops), and after it was overcome, then locked the elevator in the full nose up deflection.

My thinking, and this is raw speculation based on only a few facts available on the surface, is this: Sergio and Jim might have started experiencing elevator control interference while loitering near Mt. Rose for the start of the task. So they headed for Carson City across the Washoe valley where they would have many more opportunities for landing than in the rough and densely timbered environment around Truckee. At some point the interference got so bad that the speed could not be reduced, and the glider accelerated. At that point, a strong pull to the stick overcame the resistance, and the stick went immediately to the aft stop. Then whatever caused the initial interference locked the stick in that position.

Again, this is pure speculation with little or no intrinsic value. However, it invites the question: What would I do in that situation? How long is it prudent to troubleshoot and explore limits?

A close inspection of the burned out wreckage may reveal any unusual metal tool.

That is as I am hoping. This is a very disquieting accident, and I hope that the NTSB and interested parties arrive at a clear probable cause.

--Bob K.