"It is an outrageous act that the Iranians are going to have to be held accountable," U.S. Vice President Biden told ABC News Wednesday. "This is really over the top."
But the alleged plot was so crudely worked out that U.S. officials originally did not believe Iran was genuinely behind it, The Washington Post reported.
Though U.S. Justice Department investigators eventually linked the alleged plot to Iran's elite Quds Force, the Post said, nothing in the case had the characteristic hallmarks of that military unit. The Quds Force is the foreign terror branch of Iran's Islamic Revolutionary Guards.
Two wire transfers totaling $100,000 from Iran to an undercover agent in Mexico finally persuaded investigators the assassination plot had high-level Iranian support, officials told the Post.
The U.S. government says Manssor Arbabsiar, 56, of Corpus Christi, Texas, was allegedly working for an arm of the Iranian government when he tried to hire a Mexican drug cartel to carry out the assassination. It turned out, Arbabsiar was talking to a federal Drug Enforcement Administration informant.
While White House officials told ABC News the U.S. response would not include the possibility of an armed conflict with Iran, Biden said "nothing has been taken off the table."
The Treasury Department Wednesday imposed sanctions against the Iranian airline Mahan Air providing financial, material and technological support to the Islamic Revolutionary Guard Corps' Quds Force.
Iranian officials have denied the allegations.
"The U.S. allegation is, obviously, a politically motivated move and a showcase of its long-standing animosity towards the Iranian nation," the head of the Iranian mission to the United Nations said in a letter to the U.N. secretary-general.
U.S. officials said Arbabsiar agreed to pay $1.5 million for the assassination of the Saudi ambassador and engineered the wiring of two payments of $49,960 on Aug. 1 and Aug. 9 to an FBI undercover bank account.
Arbabsiar and Gholam Shakuri, a member of the Quds Force, were charged Wednesday with conspiracy to murder a foreign official, conspiracy to use a weapon of mass destruction and conspiracy to commit an act of international terrorism, among other charges. Arbabsiar was arrested in New York last month while Shakuri remains at large.
Prosecutors said Arbabsiar confessed to his participation in the alleged plot and provided information about the Iranian government's role.
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