Why it's so hard to stop Americans like Mohammed Hamzah Khan from trying to join Islamic State

The charges against Mohammed Hamzah Khan, a 19-year-old Chicago-area resident who allegedly tried to board a plane to join the Islamic State, illustrate how Americans' concerns about terrorism are shifting to include Americans traveling abroad to fight in foreign terrorist groups.

That concern has been mounting over the past year. Mr. Khan is the seventh American to be indicted after attempting to join IS this year, according to Karen Greenberg, director of the Center on National Security at Fordham University in New York. 

"The government has sent a very strong message that if you want to fight with ISIS you're fighting against us," she says. 

While it is unclear why Khan, of Bolingbrook, Ill., was originally stopped at the airport, he was charged Monday with violating a federal law that prohibits attempts to provide "material support or resources" to foreign terrorist organizations. 

Khan faces up to 15 years in prison and a $250,000 fine, according to a release from the US Attorney's Office for the Northern District of Illinois. 

The US has no laws forbidding someone from fighting abroad with foreign militaries, nor does it have laws preventing Americans from working with foreign groups not designated as terrorist organizations. But the US government can prosecute individuals using anti-terrorism laws. And in May of this year, the State Department labeled the Islamic State a foreign terrorist organization. 

In September, a Colorado teenager, Shannon Conley, pleaded guilty in federal court on a similar charge of conspiring to assist the Islamic State. Ms. Conley agreed to a plea deal that requires her to help law enforcement identify other individuals with similar intentions. She could serve up to five years in prison and owe a fine of $250,000.

Professor Greenberg says Khan's case shows the difficulty in prosecuting cases against people supporting "non-state actors." 

"The non-state actor issue, which has been with us since 9/11, has not been addressed head-on since the beginning of the war on terror," she says. "And it has everything to do with the difference between war and crime." 

US authorities estimate that about 100 Americans have tried to go to Syria to join groups there fighting against Syrian President Bashar al-Assad. Such groups include those backed by the US as well as the Nusra front, a group with Al Qaeda affiliations, and IS. FBI Director James Comey told "60 Minutes" that he knows of a dozen or so Americans currently fighting in Syria. 

Khan was arrested Saturday at O'Hare International Airport. He had a ticket to fly to Istanbul on Austrian Airlines. Once there, he was planning to cross the border into Syria, according to a federal criminal complaint released Monday. He allegedly waived his Miranda rights and said during the FBI interview that he had contact information for someone in Istanbul who could connect him to members of the Islamic State. 

Investigators say Khan left a three-page, handwritten letter saying, "We are all witness that the Western societies are getting more immoral day by day." Investigators also say he also expressed anger in his note that his US tax dollars were being used to kill his "Muslim brothers and sisters," a possible reference to US airstrikes against the Islamic State.

When searching Khan's home, investigators say they also found a handwritten notebook. One page appeared to show an IS flag. Another page contained a drawing of what appeared to be an IS fighter framed against an IS flag below which was written in Arabic, "Come to Jihad," according to the complaint. 

A federal judge ordered that Khan be held in custody. His detention hearing is set for Thursday.