The plot looked very suspicious. It was too stupid to belong to any government, even IRI. The guess was that this is either an MEK plot or a plot initiated by pro Ahmadinejad people to create chaos for IRI to overcome their opponents aligned with Khamenei. MEK would do something like this and spend $100K to smear IRI. They have done it in the past. I hate IRI but don't trust MEK neither.
FBI: Islamic State sympathizer charged in backpack bomb plot (AP)
from the article: Iran complains to UN over U.S. accusation of Saudi assassination plot