The X-Men were created by Charles Xavier who trained them at a private school in a mansion in upstate New York.
Donald was an unruly teenager who was trained at a private school named NYMA in upstate New York.
Many of Donald's classmates went into military careers.
Donald was trained
GITMO was expanded for this
There won't be any traces in the Department of Justice or the Court system such as PACER.
This is the US Military.
It is a separate justice system for those who serve in the military (many corrupt criminals) and for foreign illegal combatants.
Watch the film A Few Good Men to get a glimpse of how it works. It is all done by military officers. No jury. It's a Tribunal, or a Court Martial.