Is the US FBI a credible investigator for something like this?

a) I don't know that they have the right skills to track down patient zero of a disease.

b) They certainly don't have jurisdiction to do such an investigation in a foreign country, unless invited, and there's no way they were invited.

I strongly doubt we'll ever see a good answer on this, unless we get very lucky and some random research sample is looked at later that shows detection months earlier in a different location or if the Chinese did a good investigation and kept it quiet, but it leaks in a couple decades.

A timely and thorough investigation might have been able to determine an origin, but the US FBI certainly didn't do one.