Most likely brought in from illegals crossing the border.
No doubt about that. In fact, an ER doctor not too long ago was interviewed and he specifically cited the Hispanic population in CA as being spreaders. They come and go through the border, many live in multi-family homes, they are service workers, etc. I believe he said that they are by far the most hospitalized because they wait until they are extremely ill, In fear of being deported, thus working and spreading COVID.