The outbreak is attributed to the Eloy Detention Center, a privately run federal immigration detention center. This doesn’t surprise me; measles was eliminated in the U.S. in 2000, but people traveling to the U.S. (including Americans returning from foreign countries) are the biggest source of outbreaks. Disneyland was the epicenter of a measles outbreak in 2015 for just this reason.
The likely carrier in the Arizona situation was a migrant, but the problem was amplified by unvaccinated employees at the facility. The detainees have been cooperative and received vaccinations, but apparently many of the employees have been refusing or haven’t shown proof of vaccination.