I know many preppers, including some around here, do not believe the FEMA camps exist. And they very well my not, under that designation. But there are facilities that could easily accommodate large numbers of civilian, as well as military, prisoners. They might be run by civilian authorities if they are used for civilians under civil law, or by the military if they are used for real prisoners of war, or if, under martial law, it is deemed that anyone that violates martial law is to be considered a prisoner of war, rather than a civilian, they could be in camps managed by the military.
There are a few more possibilities, all dependent on what actually happens in the future. But internment facilities do exist, no matter what they are called, and who has current responsibility for them. How they are ultimately used, and by whom is yet to be seen.
Just my opinion.