Was arrested teen on his way to join ISIS?.

(CNN) -- Before authorities arrested him at O'Hare International Airport and accused him of attempting to provide aid to ISIS, a teen from the Chicago suburbs left behind a letter for his parents.

Mohammed Hamzah Khan, 19, wrote that he was leaving the United States and on the way to join ISIS, according to a criminal complaint. He invited his family to join him in the three-page letter, which authorities found in the bedroom he shared with a sibling in Bolingbrook, Illinois. But he warned them not to tell anyone about his travel plans, the complaint said.

"First and foremost, please make sure not to to tell the authorities," he wrote, according to the complaint. "For if this were to happen it will jeopardize not only the safety of us but our family as well."

Teen charged with trying to aid ISIS

A round-trip ticket was purchased for Khan from Chicago to Istanbul, authorities said.

U.S. teen allegedly tries to join ISIS

But the teen never made it to his destination.

Once he crossed security at the airport Saturday, federal agents stopped him and executed a search warrant at his home, where documents expressing support for ISIS and jihadists were recovered, the U.S. attorney's office said. Among them, according to the complaint: drawings of the black ISIS flag and a notebook including a sketch of an ISIS fighter accompanied by words in Arabic: "Come to Jihad."

Now he's charged with one count of attempting to provide material support to a foreign terrorist organization.

The teen was taken into custody without incident and made his first court appearance Monday CNN affiliates reported.

The charge he faces carries a maximum penalty of 15 years in prison and a $250,000 fine.

His parents declined to comment after Monday's hearing, The Chicago Tribune reported.

"As Khan was being led from the courtroom by deputy marshals, his father put his arm around Khan's weeping mother and sought to calm her," the newspaper said.

Letter: Teen felt obligated to 'migrate'

Details about who purchased the airplane ticket for Khan and whom he was planning to meet in Turkey were not included in the criminal complaint.

But the court document does state that the FBI has withheld many details for now, including in the document only enough to persuade a judge to criminally charge Khan.

The search at Khan's Bolingbrook home, where he lives with his parents, turned up documents allegedly written by Khan that stated his intentions.

In the letter, Khan wrote that there is an obligation to "migrate" to ISIS-controlled territory.

ISIS, an extremist Islamist group, has been fighting to take over a swath of territory in Iraq and Syria where it wants to establish a caliphate or Islamic state.

"We are all witness that the Western societies are getting more immoral day by day," Khan is wrote in the letter, according to the complaint. "I do not want my kids being exposed to filth like this."

Finally, he asks his family to join him in the "Islamic State."

During questioning at the airport, Khan waived his Miranda rights and told FBI agents that a person he met online (not identified by name in the court document) had given him the phone number of a person to call once he arrived in Istanbul.

That person, Khan told agents, was to take him to ISIS territory.

According to the complaint, Khan told authorities he was planning on being in some type of public service -- like a police force -- or providing humanitarian work or a combat role.

Turkey, the complaint notes, is a common transit point for foreign fighters from Western countries who travel to join ISIS.

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This isn't the first time authorities have accused Americans of supporting ISIS.

Last month federal authorities detailed their case against the owner of a New York food store who they accused of funding ISIS and plotting to gun down U.S. troops who had served in Iraq.

A law enforcement official told CNN that former Boston resident and U.S. citizen Ahmad Abousamra could be a key player in the ISIS social media machine that's become renowned in recent weeks for spewing brutal propaganda online -- messages meant both to terrify and recruit Westerners.

And CNN obtained tapes of American terrorists recruiting friends in the United States to join terror groups like ISIS.

ISIS has successfully recruited large numbers of foreign fighters from across the globe, including from the United States and Western Europe.

A CIA source told CNN last month that more than 15,000 foreign fighters, including 2,000 Westerners, had gone to the civil war in Syria. It was not immediately clear how many had joined ISIS and how many were with other groups opposed to the Syrian government.

The foreign fighters come from more than 80 countries, the CIA source said.

A top State Department official insisted Monday that American efforts to combat ISIS' powerful online message are working, successfully keeping disaffected youth from joining the extremist group.

"We have evidence that there are young people who are not joining because we have somehow interceded," Under Secretary of State for Public Diplomacy and Public Affairs Richard Stengel told CNN's Elise Labott on Monday.

"They're reading the messages, they're hearing the messages -- not just from us but from the hundreds of Islamic clerics who have said that this is a perversion of Islam, from the hundreds of Islamic scholars who have said the same thing."

"It's a very small cohort," Stengel said of these so-called "foreign fighter" cases originating from the United States.

CNN's Laura Koran, Michael Pearson, Shimon Prokupecz, Deborah Feyerick and Greg Botelho contributed to this report.