NTSB hearing now in Q&A session. From Hersman's briefing:
o Not caused by mechanical impact on the battery
o Not caused by external short circuit
o Event started in one cell (cell #6) and spread to other cells.

Now looking at the Boeing certification and testing in depth,
with particular attention to the special conditions imposed
by FAA in 2007 on use of Lithium-Ion batteries:
o Boeing estimated chance of smoke emission
at 1 event in 10,000,000 flight hours, however
there were two events in less than 100,000 hours.
o Boeing said that design of battery would
prevent cell-to-cell propagation but NTSB
claims that is exactly what would happend.

Interim factual report will be issued within 30 days.
FAA makes the calls on flying, not NTSB.

--bks