Interesting that the NTSB have been asked to participate, seemingly because of the origin of the engines. Not quite sure why this is legally or technically necessary - normally the contribution from the engine manufacturer is sufficient, so either the French wish to have impartial observation or there is some other reason we are not party to. Today 17:20
If the expertise of the engine manufacturer is needed by the BEA, and the engine manufacturer is in the US, then the US is entitled to acc rep status, thus the involvement of the NTSB. The legal basis for this is Annex 13
ICAO Annex 13:
5.23 Any State which on request provides information,
facilities or experts to the State conducting the investigation
shall be entitled to appoint an accredited representative to
participate in the investigation.
Also see that, under 5.18, there is a note:
Nothing in this Standard is intended to preclude the
State that designed or manufactured the powerplant or major
components of the aircraft from requesting participation in the
investigation of an accident.