It's only appropriate that the NTSB serves as the primary point of contact for matters relating to the crash investigation. NASA is acting properly by deferring all questions to the NTSB.

The events leading up to this crash appear unprecedented in commercial airliners. There have been airliner structural failures before but none involving composites that I can recall.

Composites involves large number of fibers bound together by an agent like epoxy. The structures are usually heat treated in large autoclaves. Any form of sabotage that could cause the fibers to delaminate should be pretty easy to detect. This case remains a mystery, leading me to suspect sabotage probably wasn't involved.

So, if I'm right and it wasn't sabotage, what could've caused the vertical stabilizer to break off? That's the million dollar question. Some possibilities that come to mind involve that particular plane's history.

When it was manufactured in 1988, delamination was detected in one of the 6 composite attach points. The delamination was repaired using the standard technique of applying metal plates (called doublers) and rivets. One possibility is that this repair weakened over time, leading to the failure. If this was the cause, then all aircraft using the same repair technique need to be inspected. This could potentially affect hundreds of planes.

Another possibility involves perhaps undetected damage caused by an incident where that plane encountered severe turbulence 7 years ago. The turbulence was severe enough to injure over 40 people. It's possible some damage was caused in this incident, perhaps to the repair or perhaps internal to the composites themselves. If this turns out to be the cause, then I'd want to see ultrasonic inspections on composite structures, especially those with repairs.

A scarier possibility is that there could be some form of aging problem with composite structures. This seems unlikely because there are older planes with far higher lifecycle hours and operational cycles than the one that crashed.

Perhaps a more likely possibility is some form of manufacturing defect in that particular vertical stabilizer. Back in 1989, a DC-10 airliner crashed when a manufacturing defect in the center engine (a compressor blade, if memory serves) let go and took out the plane's hydraulic systems. The defect turned out to be a very tiny void in one of the blades that weakened the part. It's entirely possible that some manufacturing defect, perhaps quite subtle, was built into that vertical stabilizer.