I wonder why they haven't sought Josh for questioning yet? I wonder if this is a sign that he is not available in this country? Maybe Carlos is just lower hanging fruit?
We don't really know what "they" have or haven't done, do we? We only learned about Carlos because he managed to get sued for refusing to talk.
Confirming what I said Suchmoon: The regulator likely subpoenaed Carlos Garza because the whereabouts of his brother, Josh, are unknown.