CHICAGO -- A 19-year-old American left a letter expressing disgust with Western society before trying to board an international flight in Chicago, the first step in his plan to sneak into Syria to join the Islamic State group, according to a federal criminal complaint released Monday.

Mohammed Hamzah Khan, who lived with his parents in the Chicago suburb of Bolingbrook, was arrested Saturday at O'Hare International Airport trying to board a plane on the first leg of connecting flights to Turkey, which borders Syria. He is charged with attempting to provide material support to a foreign terrorist group, which carries a maximum 15-year prison sentence.

Investigators said Khan left a three-page, handwritten letter in his bedroom for his parents that expressed anger over his U.S. taxes being used to kill his "Muslim brothers and sisters," an apparent reference to a bombing campaign against Islamic State militants.

"We are all witness that the western societies are getting more immoral day by day," he wrote, then signed letter, "Your loving son," according to court documents.

Khan appeared in a federal court Monday in orange jail clothes, calmly telling a federal magistrate he understood the allegations. As marshals led him away in handcuffs, the slight, bearded young man turned to smile at his parents - his father putting his arm around Khan's weeping mother.

About a dozen Americans are believed to be fighting in Syria, while more than 100 have either been arrested on their way to Syria or went and came back, FBI Director James Comey said recently without offering details.

Khan sought to fly Austrian Airlines to Istanbul by way of Vienna when customs officers stopped him going through security at O'Hare's international terminal. While FBI agents interviewed him there, investigators searched his home.

It's unclear why authorities stopped Khan. Prosecutors, Khan's federal defender attorney and his parents didn't comment after Monday's hearing.

In the letter found by FBI agents, Khan also pleaded that his parents not contact authorities. Other documents found during the search of his home included a notebook drawing of what appeared to be an armed fighter with an Islamic State flag and the words "Come to Jihad" written in Arabic, according to the complaint.

Also found were drawings with arrows indicating where Khan might cross the border into Syria from Turkey.

Khan allegedly told FBI agents that an online source gave him the number of a person to contact when he got to Istanbul who would lead him to Islamic State members. When asked what he would do once in territory controlled by the Islamic State, Khan allegedly said he would "be involved in some type of public service, a police force, humanitarian work or a combat role," according to the complaint.

Khan was ordered to remain jailed until at least a detention hearing Thursday. Prosecutors indicated they would ask he stay behind bars pending trial.

At a two-story house believed to be his family's home, no one would address reporters outside. But neighbor Steve Moore, 31, described Khan as a soft spoken and polite, saying the young man his family were always friendly and quick to say hello.

Another young man from the Chicago area also is accused of trying to join militants in Syria. Abdella Tounisi, of Aurora, was arrested last year at O'Hare when he was 18. He has pleaded not guilty to seeking to provide material support to al Qaeda-affiliated Jabhat al-Nusra in Syria.