I believe the NTSB will be party to the investigation because it is the investigating body from the nation of manufacture and company registry. The Japanese will participate because the accident took place in their country.
If I remember correctly all three of these parties (nation of manufacture, operation and accident site) are party to the investigation but that the Japanese lead the investigation because it took place on their soil. All that being said, even if the Japanese lead (they will probibly give the NTSB the honors since they aren't as experienced) the NTSB will issue their own final report.