Mr. Ithawi, described by the Iraqis as a top aide to the Islamic State leader Abu Bakr al- Baghdadi, had been in charge of fatwas, or religious rulings, in the Islamic State’s so-called caliphate. He was also in charge of the education curriculum, and was a member of the body that appointed security and administrative leaders for the Islamic State’s territory, which had included large parts of Iraq and Syria.
He had been living in Turkey with his Syrian wife under his brother’s identity, one of these officials said.
The Iraqis sent the Turks an intelligence file they had amassed on Mr. Ithawi, and the Turkish security forces arrested him on Feb. 15, and extradited him to Iraq, this official said.
Iraqi and American intelligence officials then spent weeks interrogating him, learning the details and whereabouts of other ISIS leaders in hiding, the officials said.
The American-led coalition used this information to launch an airstrike in mid-April that killed 39 suspected Islamic State members near Hajin, in the Deir al-Zour district of Syria, the second official said.
The joint Iraqi-American intelligence team then set a trap, according to these officials. They persuaded Mr. Ithawi to contact several of his Islamic State colleagues who had been hiding in Syria and lure them across the border, the officials said.
The Iraqi authorities were waiting, and arrested the group soon after they crossed the frontier, the officials said.
Those arrested included Saddam al-Jammel, a Syrian who had been the head of the Islamic State territory around Deir al-Zour, and Abu Abdel al-Haq, an Iraqi who had been the head of internal security for the group. Two other Iraqis were also arrested, the officials said.