Tehran?
Maybe Iran had a melt down, or internal accident.
I-131 is usually seen in a gaseous state, so imagine putting a pressurized canister in the back of a trunk, then driving down the autobahn or other such roads, dispersing as you do.
Makes point of release computations, unless some has contaminated objects on one side of the road as opposed to the other. In that case, you have a what, where, when, and how..... but no “who.”
If not revealed by the point of emission study, then an analysis of trace elements may indicate a source and/or a reactor in which it was produced.
For example, at PANTEX, they were working with some Hanford Plutonium. They could even tell in which reactor it was created, and where in that reactor!
Looked at the winds aloft at 5000 and 25,000 feet.
Could be.
No- Japan.