The cases are reported to be in a Mennonite community (they do not vaccinate, but I believe they also do not send children to public schools and mostly keep to themselves).
SO, the question is, how was that community infected? In the US, we have pretty much eradicated (or have herd immunity for) measles. They need to trace back to an (unvaccinated) carrier who had contact with that insular community. Where would the most likely cross vector be? Food Pantry? Farmer’s Market?
(and yes, I agree that it is probably a “newcomer” i.e., an illegal immigrant)
I think it is quite a coincidence that this measles outbreak is happening just weeks after RFK Jr. became director of HHS. What better way to discredit him and Trump than to infect a religious group who does not believe in vaccinations?