The United States believes that a U.S. strike in Syria has killed Mohsin al-Fadhli, the leader of a group of al Qaeda militants known as Khorasan, a U.S. official told Reuters on Wednesday.

“We believe he is dead,” the official said, speaking on condition of anonymity.

U.S. officials have said the al Qaeda-linked group posed an immediate threat to the West and had been nearing the execution of a plot against U.S. or European targets.