“The Iranians watch the Saudis roll tanks in Bahrain, and they see a key ally in Syria going down, so they step up the Quds Force,” one senior administration official said. He referred to Saudi military assistance to the Sunni monarchy of Bahrain, whose majority population shares the Shia Islam of Iran.

Iran has many trusted networks in the Middle East and has often used the Lebanese militants of Hezbollah as a proxy. But it has far fewer agents in the United States, which might have forced it to look to a far riskier proxy for the plot, officials said.

American investigators have speculated that the Iranian-American accused in the scheme, Mansour J. Arbabsiar, who lived in Texas on the Mexico border, may have convinced a cousin, a senior Quds official, that he could recruit a member of one of Mexico’s notorious drug cartels to carry out the killing.

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.

In a protest letter denying the American charges late Tuesday, Iran’s ambassador to the United Nations, Mohammad Khazaee, referred pointedly to the assassination campaign. “Iran has been a victim of terrorism,” he wrote, “a clear recent example of which is the assassination of a number of Iranian nuclear scientists in the past two years carried out by the Zionist regime and supported by the United States.”

An American official said of Iranian officials that “certainly their publicly expressed anger at the death of some of their scientists could have been part of their calculation.” But the official said the United States government had no specific evidence to support that theory.