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.