The defendant, Mansour J. Arbabsiar, a used-car salesman from Texas, had been accused of conspiring to hire assassins from a Mexican drug cartel for $1.5 million to kill the ambassador, Adel al-Jubeir.

At the time the charges were announced, Attorney General Eric H. Holder Jr. said the plot had been “directed and approved by elements of the Iranian government and, specifically, senior members of the Quds Force,” part of Iran’s powerful Islamic Revolutionary Guards Corps.