Since I deal with transporting goods rather than people, I don't know off hand in what manner FMCSA regulations apply here. While I seldomly encounter tour groups in the field, I don't recall seeing a DOT/ICC number OR an IFTA sticker on the sides of any tour vans that I have seen. If they do not require a DOT/ICC number (and are not applicable to IFTA), then I'm not certain FMCSA would potentially have jurisdiction here. HOWEVER, if they do have DOT/ICC number, then they are absolutely under FMCSA jurisdiction. I believe this will be the key piece of information that will determine who to reach out to from the regulatory side. It would almost certainly be easier to point someone to FMCSA than it would be to find an applicable state agency, either in the state based out of, or the state then incident occurred in, or both.
Bear in mind, I don't have any first hand knowledge, so there are a lot of "ifs" in my statement. But those are my thoughts as someone who has worked a DOT regulated job for several years now.