The most fascinating part of U.S. Attorney General Eric Holder's revelation today was that of one of the charges being "conspiracy to use a weapon of mass destruction".
Since the Iranians were apparently hoping to pay a member of a Mexican cartel 1.5 million dollars (to carry out the assassination?) it begs the questions what WMD did they believe the Mexican gang member could obtain? My guess is this is some sort of chemical weapon, sarin perhaps. Then an assassination could be a blanket attack over a larger area that includes the open air location of the Saudi ambassador.
But we shall see, as and when more details emerge. Since WMD carry with them a normative prohibition this works as excellent propaganda - the Iranians using outlawed weapons of indiscriminate killing in order to carry out an assassination.