One of the fathers of modern radical Islam spent six months in Northern Colorado in 1949 and went home to Egypt where he authored essential writings for today's jihadists.
Jordan MacTaggert and Levi Shirley died over the summer in Syria.
They're among dozens of Americans who have traveled to Syria to fight. A Colorado congressman helped bring their bodies back home.
Last week Kurdish forces announced that Levi Shirley died while fighting with them in Syria. More than 100 Americans have volunteered to fight ISIS outside the U.S. military.
The attack in Paris has prompted questions about how vulnerable the U.S. is. A former FBI official and a terrorism scholar assess the risks.