james t. booker , 20 , of topeka , kansas , was arrested following fbi sting .
his plan to join army was stopped last year after extremist facebook post .
he faces terrorism charges , including using a weapon of mass destruction .
booker arrested outside fort riley 's supposed ` secret gate ' with car bomb .
imam says that suspect was mentally ill .
alexander blair , 28 , arrested for failing to inform authorities about plan .