This is the case of Ali Saleh Kahlah al-Marri:

This is not a case of someone being detained on a battlefield or even overseas, nor is it the case of someone who entered the country illegally. He was in the U.S. legally and was detained while sitting at home. And just as he was about to start his criminal trial, the President essentially cancelled the trial and ordered him detained indefinitely and incommunicado

Regarding legal issues, this is some of the impact of the Unlimited Detention and Torture Act (also known as the Military Commissions Act). 

A complete betrayal of the legal underpinnings of the Constitution.  A complete betrayal of any type of ethical system.

It looks like one of the first things Democrats will do, is attempt to address this law, removing or altering some of the more egregious elements.

Let's all pray for success in this.