TEHRAN — Iranians are as obsessed as Americans these days with the black-clad gangs roaming Iraq and Syria and killing Shiites and other “infidels” in the name of Sunni Islam. At the supermarket, in a shared taxi or at a family gathering, conversations often turn to the mysterious group, the Islamic State in Iraq and Syria, and how it came to be.

And for most Iranians, the answer is obvious: the United States.

“Come on, you know who has created ISIS,” said the supermarket owner, winking his right eye. “Admit it,” demanded the taxi driver, hitting his steering wheel to make his point. “It is so obvious!” concluded the talkative uncle at the birthday party.

ISIS, Iranian leaders have been saying for a long time, is made-in-the-U.S.A., a tool of terror intended by the world’s superpower to divide and conquer the energy-rich Middle East and to counter the growing influence of Iran in the region.

Iran’s supreme leader, Ayatollah Ali Khamenei, has often said that he believes ISIS was created by the United States as a way to regain a foothold in Iraq and to fight President Bashar al-Assad of Syria, an ally of Iran.