undergoes military/terrorist training (thereby forfeiting his American citizenship);
No.
You may lose your citizenship under some pretty dire circumstances, but affiliating with a group, and not of a state, is not one of them.
Did this guy kill any Americans? Was he part of the 9/11 conspiracy? No. He received training (perfectly legal) joined a group (perfectly legal), was in the US (perfectly legal), and had $10526 in cash (illegal by $527). The fact that he hates America is perfectly legal. If it were not, entire cities would be wiped off the map (Berkeley, Cambridge, Seattle, most of Marin County-CA, etc). He has a right to walk the streets and burn our flag, chant 'death to America,' and say really mean things about GWB. That is all protected under the First Amendment (provided he does not take out advertising within 60 days of a Presidential election).
Did he have bomb materials? Did he kill any Americans?
Sorry, but he must be charged within 48 hours, or released.
"We need some new system to handle suspected terrorists, outside the standard criminal and military paradigms that have proven a worse fit with each successive case. We need some forward-looking system, based on the likelihood of future danger.
We need a Bureau of Precrime."