A 19-year-old Muslim in Chicago is curious about the Islamic State. He’s thinking about sneaking off to Syria to join the militants, and he’s already gotten passports for himself and his 17-year-old sister and 16-year-old brother.

Maybe he’s still on the fence. Will he really go through with it? His Indian-immigrant parents keep a close eye on the family computer, trying to keep their children in a “cocoon” safely away from what they see as the negative influences of American culture.

But they haven’t quite realized that their son’s phone is also gateway to the whole world. And that world is filled with propaganda promoting the Islamic State in Syria and Iraq as a family-friendly paradise.

So at 7:31 p.m. on Aug. 7, 2014, Mohammed Hamzah Khan, using a pseudonym and a Twitter handle bristling with teenage bravado, “lionofthed3s3rt,” sends out a tweet. It is a direct message to a man in London he apparently thinks can answer his questions: Mizanur Rahman, known on Twitter as "abu_baraa1," one of the most prominent online promoters of the Islamic State.

Using a mix of Arabic phrases and English, dotted with text-speak abbreviations, dropped words and typos, Khan sent this:

That translates to: “Brother, what do you say to people who claim that preaching is more important right now since all the Muslim people aren’t ready to live in a caliphate?"

But in Khan’s case, it amounts to a more basic question: The teenager is asking whether it’s okay for him to stay in Chicago and spread the word about the caliphate, or if he needs to actually go there.

It is the tweet of a seeker, someone who might be nervous about the enormity of his plans to abandon his parents and his life and move halfway around the globe into the middle of a war zone.

It’s unclear why Khan reached out to Rahman. But U.S. and U.K. authorities say he is an Islamic State recruiter, who sets himself up as an online “beacon” offering inspiration and religious justification for anyone thinking about joining the militants.

Rahman denies that allegation that he is a recruiter, saying that he is not affiliated with the Islamic State, and his thousands of tweets, Facebook posts and YouTube videos simply praise Islam and the idea of a caliphate, or a society ruled by sharia law.

After last Friday's Islamic State attacks in Paris, Rahman said in a telephone interview that the bloodshed was "an inevitable consequence" of France's participation in coalition airstrikes against the Islamic State. "In war, people bomb each other," he said. "It's something which any sane, rational person would expect."

In an interview last month in London, Rahman was shown a print-out of his tweets with Khan. He explained their religious significance, and noted that he never told Khan to go to Syria, and that he was not trying to urge him to go.

But eight rapid-fire tweets he sent back to Khan cast light on what authorities say is the smooth and savvy behavior of someone who knows exactly where the line is between free speech and the act of inciting someone to join a radical Islamist organization.

Here is his first response to Khan’s query, posted at 7:50 p.m.:

Rahman is saying that establishing a caliphate is “fard,” or a religious duty commanded by God. He says that even if all Muslims aren’t ready, that is not an excuse not to establish a caliphate and swear "bay’ah," or an oath of allegiance.

Saying “time limit = 3 days” is a historical religious reference. Rahman says that under Islamic law and tradition, if the caliph, or leader of the caliphate, dies or is no longer available, it is required that a new one be named within three days.

Khan quickly retweeted Rahman.

Rahman’s next tweet, posted less than a minute later, continues the same thought:

Rahman said it is “wajib,” or necessary, to declare a caliphate and swear allegiance immediately and that waiting a fourth day is a sin. When he says, “it’s been 90 yrs already,” he is referring to the fact that the world’s last caliphate, the Ottoman Caliphate, was abolished in 1924.

“Since then, Muslims never had any authority of their own,” Rahman said in the interview. “We were always under the thumb of another dictator or tyrant or United Nations or Western occupation.”

That's a key part of Rahman's argument: He claims that believing in a caliphate is not the same as supporting the militant group that established it. Authorities have called that argument absurdly self-serving.

One minute later, Rahman tweeted:

Here Rahman stresses to the teenager that while preaching about Islam is “fard,” or every Muslim’s duty, it is not enough. He says Muslims must also try to establish a global caliphate as foretold by the Prophet Mohammad. And, he says, they must perform "jihad," a word that means different things to different people. Rahman says he is referring to the personal struggle to maintain and defend Islam. But "jihad" is also often used as shorthand for an armed assault against non-believers.

In this case, all that matters is Khan’s interpretation, which is the point that law enforcement officials make. On the face of it, his words can appear totally benign. But to a young Muslim searching for meaning, trying to make sense of his religious duties in a world that he sees as hostile to his religion -- as Khan did -- those words can seem like a command.

To that tweet, Rahman instantly added:

Here Rahman’s words suggest that no one should feel compelled to move to Syria or Iraq to join the caliphate if that would be too much of a burden on them. On the other hand, Khan’s interpretation is still the key: Did he feel that he had the capability to upend his life and move to the Middle East to join the Islamic State? It's a simple sentence that offers legal cover to Rahman, but potentially offers a subtle call to action to an angry, agitated young man trying to find his path.

More than a half hour passes before Rahman tweets again, according to his Twitter account, which is still active.

At 8:25 p.m., he sends this message to Khan:

He is repeating his earlier call for allegiance to the caliphate, only this time with more urgency -- suggesting that the caliphate could fall tomorrow. As he has throughout the conversation, Rahman uses colloquial text English (“u” for “you,” and the “=” sign), which officials say show his smart communication skills and cultural connection with young people who make up the majority of Islamic State recruits.

One minute later:

Rahman turns up the heat: Even one day of loyalty to God's wishes is better than a lifetime of "jahiliyyah," or ignorance of God's guidance.

"He is supplying the ideological justification for joining a group like this," said Charlie Winter, a researcher at the Quilliam Foundation in London, who reviewed the Twitter exchange. "This is effectively a key that unlocks that door."

A minute later, he tweets:

Analysts who have looked at this exchange said Rahman, by starting the tweet with "No," appears to be answering a different question from Khan, but the Twitter history has no record of a second tweet from Khan. It is possible that Khan tried to cover his tracks by deleting it. Rahman also said he believed there must have been another question, which he said was probably something like, "Do you have to go to Syria to give allegiance, or can you do it from far away?"

Here, Rahman's answer tends to support his argument that he's not urging Khan to travel to join the Islamic State. However, he introduces the word "obedience" into the conversation for the first time. And in that, the analysts see mischief: If you tell a young man to obey the teachings of the caliphate, isn't that subtly urging him to obey their specific call for all able-bodied Muslims to come to the caliphate to join the struggle?"

A minute later, the final tweet in the sequence:

By mentioning the word "him," Rahman changes the dynamic. There is only one "him" when it comes to the caliphate: Abu Bakr al-Baghdadi, the caliph, or leader. Baghdadi is also the leader of the Islamic State, a man so ruthless and pitiless that al-Qaeda has distanced itself from his barbaric violence.

Urging someone to obey Baghdadi is unambiguous. In audio messages released by the Islamic State, Baghdadi has commanded: "There is no excuse for any Muslim not to migrate to the Islamic State... joining is a duty on every Muslim."

Less than two months after his Twitter exchange with Rahman, on Oct. 4, 2014, after attending pre-dawn prayers with his father at their suburban Chicago mosque, Khan snuck out of his family home and caught a taxi to O'Hare International Airport with his 17-year-old sister and his 16-year-old brother.

The FBI, which monitors radical social media accounts, was waiting at O'Hare. Last month Khan pleaded guilty to providing material support to a terrorist organization, and he could face up to 15 years in prison.

The morning he left home, Khan left a note for his parents explaining his actions:

“An Islamic State has been established and it is thus obligatory upon every able-bodied male and female to migrate there,” he wrote.

His heartbroken parents said they had no idea where he had gotten such ideas.