Hercules Poirot and Miss Marple can't figure it out.
Sherlock Holmes is stumped.
One of those "Murder on the Orient Express" jobbies with altogether too many suspects.
Iran blames the US and Israel, of course.
There's certainly been an undeniable amount of animosity expressed there to the point where both nations have rattled sabres quite dramatically in the last couple of years.
The BBC infers that he was really killed by Iranian hardliners who considered him an enemy despite referring to him in their media as a "revolutionary".
One university official said Mr Mohammadi was not a political figure. But other reports said his name appeared on a list of academics backing opposition leader Mir Hossein Mousavi before the 2009 presidential election.
Maybe anti-government forces within Iran itself?
Some conservatives have suggested that the People's Mujahideen Organisation - a banned militant group opposed to the Tehran government - was involved. The group denied the accusation.
Who to believe?
No-one has claimed responsibility for the blast and at this stage there could only be speculation as to possible motives for the attack, correspondents say.
Spud speculates that there is not enuff evidence available at this time to say anything with any degree of certainty.
Worth keeping an eye on though.
Be Well.
