Coalition-backed forces in northern Syria have captured a 28-year-old United States citizen who is believed to have been fighting for the Islamic State, one of only a handful of Americans to be apprehended on the battlefield, officials said.
Ibraheem Musaibli, who is from Dearborn, Mich., was seized by the Syrian Democratic Forces this month as he tried to escape the Middle Euphrates River Valley, where the American-backed militia has been working to root out one of the last pockets of Islamic State control.
Mr. Musaibli was transferred to a holding facility, where he was identified as a member of the militant group by another detainee, according to one administration official briefed on the arrest.
The authorities have set in motion a plan to bring him to the United States for prosecution, along with an Indiana woman whose husband was an Islamic State member and who has also been detained.
The case is a test of America’s ability to legally pursue suspected members of the terrorist group captured on the battlefield, a goal that has proved elusive in recent months. Last September, the same militia captured another American at a checkpoint in the battle zone, but the Justice Department decided there was insufficient admissible evidence to prosecute him.
In the case of Mr. Musaibli and the other detainee, identified as Samantha Elhassani, the authorities are banking on the fact that the two have already been indicted in sealed court proceedings, according to authorities. The details of their capture were confirmed by officials from three government agencies, all of whom requested anonymity in order to discuss sensitive information.
Mr. Musaibli is expected to face charges of supporting the terrorist group, also known as ISIS.
Spokesmen for the State, Justice and Defense Departments, and for the F.B.I., declined to comment.
The authorities say that Ms. Elhassani traveled to Syria with her husband and children. When her husband was killed fighting for ISIS, they say, she and her four children, the youngest of whom is 8 months old, made their way to a refugee camp guarded by the Syrian Democratic Forces, where she identified herself to local authorities.
Ms. Elhassani’s 10-year-old son once appeared in an ISIS propaganda video pledging to carry out attacks against the West, raising fears that he had been radicalized, officials said. She and her children will be transported on the same military flight carrying Mr. Musaibli back to the United States, they said.
Only a few dozen Americans are known to have traveled to Syria to join the Islamic State.
A database maintained by George Washington University’s Program on Extremism has identified 71 American citizens who traveled to either Iraq or Syria to join the jihad. Of these, at least 24 have been killed. The whereabouts of another 29 is unknown; 18 either returned to the United States or are in jail.
Mr. Musaibli had maintained a low profile and did not appear in the George Washington database, although he was known to law enforcement officials.
As a teenager in Michigan, he dropped out of high school to help his father run a perfume store, according to his younger brother. He married, had a son and eventually moved to Yemen.
Officials believe he traveled to Syria in approximately 2015.
Contacted through Facebook Messenger, Mr. Musaibli’s younger brother Abe Musaibli, expressed disbelief at the thought that his sibling had joined ISIS. “My brother is a saint,” he said. “He wouldn’t hurt a fly.”
Abe Musaibli said he was not aware that his brother had been taken into custody. The last time they spoke, he said, was two months earlier.
Ibraheem Musaibli sent a series of text messages to his family after leaving for Syria in which he confirmed his intention to join the Islamic State, said two officials from different agencies who were briefed on the matter.
In the years that followed, he became disillusioned with the militants and sought a way out, the officials said, and his family reached out to the F.B.I. A brief negotiation ensued, said one official, in which the F.B.I. offered to bring him back to the United States in exchange for him turning himself in. He refused, and the negotiations broke down, one official said.
Mr. Musaibli’s capture is only the second known instance of a suspected male American ISIS member being captured in Syria.
The first man was apprehended in September at a checkpoint on an active battlefield, adjacent to territory controlled by ISIS, according to court filings. They identify him only as “John Doe.”
In his possession was a thumb drive containing internal Islamic State records, including bomb-making materials, court records show. His name, moreover, appears on an internal ISIS intake form, the detailed application that newcomers to the group are asked to fill out upon arrival in ISIS-controlled territory, the records say.
He has since claimed that he went to Syria intending to be a freelance journalist, but was imprisoned by the terrorist group and agreed to work for them as a condition of getting out of jail.
After the Justice Department decided that it lacked sufficient evidence to prosecute him, the military proposed releasing him back in Syria. He is fighting that plan in court.
The John Doe case is emblematic of the challenges that Western democracies face when their citizens are captured fighting for ISIS.
So far, Britain, France and other European nations have refused to take back the majority of their citizens being held in prisons in Syria, fearing that the cases against them would fall apart under the normal rules of criminal prosecution.
In Canada, an Islamic State member who succeeded in slipping past airport security and returning home has not been charged with any crimes, even after he gave a detailed interview to The New York Times describing how he carried out executions on behalf of the militants.
The John Doe case has made military officials leery about taking custody of other detainees. But if Mr. Musaibli is facing a sealed indictment, as officials have indicated, that could make prosecution easier.
“The difference is that here you have a case that is already set up,” said Seamus Hughes, the deputy director of George Washington’s Program on Extremism.
“That is a very different animal than picking up someone on the battlefield who is nowhere in the system,” Mr. Hughes said, adding, “If he is already indicted, then you have enough to build a case, because you had enough to build the indictment.”
Adam Goldman and Karam Shoumali contributed reporting.