“It would be our assessment that this kind of operation would have been discussed at the highest levels of the regime,” said a senior American official, who was not authorized to speak publicly about the government’s analysis.

American officials offered no specific evidence linking the plot to Iran’s most senior leaders. But they said it was inconceivable in Iran’s hierarchy that the leader of the shadowy Quds Force, Maj. Gen. Qassim Suleimani, was not directly involved, and that the country’s supreme leader, Ayatollah Ali Khamenei, was not aware of such a plan…

The officials said the plot might indicate a shift to a more combative Iranian foreign policy toward Saudi Arabia and the United States. The United States has brought international pressure on Iran’s nuclear program, and Iran and Saudi Arabia have long waged proxy battles for influence in the Muslim world…

One provocative theory that American officials are considering is that the assassination was intended as retaliation for the killing of several Iranian nuclear scientists during the past two years. Those deaths are widely believed to have been the work of Israel, with tacit American approval, to slow Iran’s progress toward a nuclear weapon.