US Attorney General Eric Holder said the US would hold Iran accountable.
Two people, including a member of Iran’s special operations unit, were charged in New York federal court.
Holder said the bomb plot was a flagrant violation of US and international law.
“We will not let other countries use our soil as their battleground,” Preet Bharara, the US attorney in Manhattan, said at a news conference in Washington with Holder and FBI director Robert Mueller.
Manssor Arbabsiar, a 56-year-old US citizen who also holds an Iranian passport, was charged with Gholam Shakuri, whom authorities said was a Quds Force member.
Mueller says many lives could have been lost in the plot to kill the ambassador with bombs in the US.
Holder said the US government would be taking unspecified action against the Iranian government as early as today.
Asked whether the plot was blessed by the top echelons of the Iranian government, Holder said the Justice Department was not making that accusation.
Arbabsiar unknowingly hired an informant from the Drug Enforcement Administration to carry out the plot, prosecutors said.
Posing as a member of a Mexican drug cartel, the informant met with Arbabsiar several times in Mexico.
The price tag was $1.5 million and Arabsiar made a $100,000 down payment.
Bharara said no explosives were actually placed, and no one was in any danger.
Shakuri remains at large. Arbabsiar was arrested on September 29 at New York’s JFK International Airport.
He was scheduled to appear in federal court today. Prosecutors said he faces up to life in prison if convicted.
Prosecutors said that Arbabsiar has confessed to his participation in the murder plot.
President Barack Obama was first briefed on the plot in June, said White House spokesman Tommy Vietor.