The BBC's Tehran correspondent, Jon Leyne, looks at the questions raised by the death of an Iranian physics specialist in a bombing on Tuesday.
Dr Mohammadi was killed as he was leaving his home, media say
According to the Iranian media, Masoud Ali Mohammadi was a nuclear scientist, assassinated by counter-revolutionaries, Zionists and agents of the "global arrogance".
The implication is clear - his killing was a Western plot to sabotage Iran's nuclear programme.
But from the start, there were many curious anomalies.
Iranian media were unusually quick off the mark to report the killing, to show television pictures, and to give the sort of details that usually only emerge after hours, days, or weeks in this secretive state.
The state broadcaster described Dr Mohammadi as a "committed and revolutionary" professor - a strange tribute to emerge so soon.
He was also described as a nuclear scientist.
Official and semi-official media blamed counter-revolutionaries, Zionists and agents of the "global arrogance" - Iran's usual term for its enemies in the West.
And a reporter for a government-owned newspaper told the BBC the scientist's death was likely to be a setback for the Iranian nuclear programme - another strange comment in a country where defiance is usually the rule.
But scientists in Britain and the United States pointed out that, from his substantial body of published research, Dr Mohammadi was most unlikely to have been working on Iran's nuclear programme.
His expertise, they said, was in another field of physics altogether - quantum mechanics.
Meanwhile, the Iranian opposition highlighted his name on a list of 240 academics who had pledged their support to the leading opposition candidate, Mir-Hossein Mousavi, in the disputed presidential elections.
This is not the sort of action calculated to bring such a fulsome tribute from the government as it battles Mr Mousavi's supporters.
So this all leaves more questions than answers.
One possibility is that Iran's opponents simply attacked the wrong target, mistakenly believing they could harm the nuclear programme.
Yet it was the official media that named him as a nuclear scientist.
The opposition will suspect it was a government attack, designed to kill one of their supporters and intimidate others.
Certainly the suggestion foreign agents penetrated into the heart of Tehran, with the current intense security, raises deep suspicions.
But why the Iranian government would target Dr Mohammadi in particular is another unanswered question.
If he was an opponent of the government, he was certainly not a leading one.
Whatever the precise circumstances of his death, the opposition will fear this will be another reason for cracking down on their activities.
If that is the case, expect more arrests and government warnings in the coming days.
And the killing will do little to help the already faltering dialogue between Iran and the West over the nuclear programme.
Representatives of six major world powers meet in New York on Saturday to discuss the current impasse in the nuclear talks.
The West will be pressing speedy adoption of a new set of sanctions.
One thing all those round the table should be able to agree is that Iran is becoming an increasingly complicated country to understand and to negotiate with, as the domestic political situation and the nuclear controversy become ever more closely intertwined.