Mr. Nur’s immigrant parents most likely did not see that by late March of last year, he was posting a quote from Anwar al-Awlaki, the late American recruiter for Al Qaeda: “We are fighting for truth and justice and you (americans/westerners) are fighting for oppression and worldly gain.”

Somehow “you” had become his fellow Americans. “We” were the jihadists. It was a few weeks after Mr. Nur and Mr. Yusuf had begun visiting a new mosque in Bloomington, outside Minneapolis. It was there that they became interested in ISIS, which was not yet associated with the videotaped beheadings of Westerners that would come later.

A passport specialist, suspicious when Mr. Yusuf applied for an expedited passport and seemed vague about its purpose, alerted the F.B.I. On May 28, agents intercepted Mr. Yusuf at the Minneapolis airport and kept him from boarding. They found that the blue Volkswagen Jetta that had dropped him off belonged to the boyfriend of Mr. Nur’s sister. But when they searched for Mr. Nur late on May 29, they learned he had left the country hours earlier.

Word that he was missing was a blow for the city’s large Somali community. “It went out like wildfire that he had left and nobody knew where he had gone,” said Abdirizak Warsame, a friend and, like Mr. Nur, a 2013 high school graduate. They had played basketball together just a few days earlier. “I had no clue,” he said.

The next day, May 30, Mr. Nur’s older sister, Ifrah, walked to the police station near the family’s apartment to report his disappearance. Over the next day or so, she managed to reach him in Turkey via messages on Facebook and an app called Kik. Their exchanges are recorded in court papers.

She challenged him, saying that their father “went into shock” and telling him that “going to kill poor people is not the answer.” She made an emotional appeal: “respond to me I love u and can’t live knowing this.” He gently put her off, saying that “if I didn’t care I wouldn’t have left but I want jannah” — paradise — “for all of us.”

During his early months in Syria, Mr. Nur was upbeat and posted frequently online. He was teaching English (“Got a Full Class”), trading tips on handguns and pistols, expressing excitement about fighting Kurdish forces (“Never Felt So Hyped”) and praising the “amazing brothers” with him. On Aug. 7, he answered questions on the website Ask.fm, including some from angry friends in Minneapolis.