One would think that if a substantial number of United States citizens were on board, that there will be significant US involvement in the investigation.
However, at this point, there seems to be some speculation on how many US passengers there were:
1. Joe Biden said "There may have been American citizens on board"
Vice President Biden speaks at Cobo | News - Home
2. Other posters: "There is supposedly a large number of Americans dead. 23 is rumored." (A few posts back). I don't know from where this number originated.
3. The official Malaysian Airlines manifest report, yet incomplete, does not include any Americans, but 41 unverified:
http://www.malaysiaairlines.com/my/en/site/mh17.html