The Justice Department scheduled an announcement at 1800 GMT on what it called “significant national security enforcement action."
News reports said the plot included the assassination of the Saudi Arabian ambassador to the United States, with a bomb and subsequent bomb attacks on the Saudi and Israeli embassies in Washington.
The case, called Operation Red Coalition, began in May when an Iranian-American from Corpus Christi, Texas, approached a US informant seeking the help of a Mexican drug cartel to assassinate the Saudi ambassador, according to counter-terrorism officials.
The Iranian-American thought he was dealing with a member of the feared Zetas Mexican drug organization, according to agents quoted by ABC.
The report said bombings of the Saudi and Israeli embassies in Argentina were also discussed.
Reports said the Iranian-American, identified by federal officials as Manssor Arbabsiar, 56, claimed he was being “directed by high ranking members of the Iranian government," including a cousin who was “a member of the Iranian army but did not wear a uniform."
Counter-terrorism officials said they believe the cousin may be part of the special operations unit of the Revolutionary Guard, the Quds force, US media said.
According to the reports, US officials said Arbabsiar met twice in July with the informant in the northern Mexico city of Reynosa, and negotiated a $1.5 million payment for the assassination of the Saudi ambassador.