His due process rights were "if he protests that he is a citizen, then they must make effort to determine if that is the case with haste, and release him if it

His due process rights were "if he protests that he is a citizen, then they must make effort to determine if that is the case with haste, and release him if it checks out". He did not assert citizenship, they did not violate due process rights. No other process is due him. Anyone claiming that it's a violation of due process rights doesn't know what they mean, though for a moment I had hope for you... you sort of described them, or at least hinted at them, but you're unable to apply what you described to this situation. Due process rights can't and won't keep non-citizens in this country if they are trying to avoid deportation, and shouldn't ever. Non-citizens only have the revocable-for-any-reason-or-none privilege of visiting, and that only if they get the visa.