Dr Mohammadi was killed as he was leaving his home, media say
The US has dismissed allegations by Iran that American and Israeli agents were responsible for the killing of a physics professor in Tehran.
The US State Department described the accusation as "absurd".
Physicist Massoud Ali Mohammadi died in a bomb blast on Tuesday, Iranian media reported, describing him as a nuclear scientist and a government supporter.
But later reports suggested he was not involved in Iran's nuclear programme and had signed an opposition petition.
Dr Mohammadi, 50, was killed by a remote-controlled bomb near his home.
Iranian TV quoted security officials at the scene as saying the equipment and system of the bomb used in the attack had been related to a number of foreign intelligence agencies, particularly Israel's Mossad.
Jon Leyne, BBC Tehran correspondent
The original story seemed straightforward. According to Iranian media, Masoud Ali Mohammadi was a nuclear scientist, assassinated by counter-revolutionaries, Zionists and agents of the "global arrogance".
The implication was clear - it was a Western plot to sabotage Iran's nuclear programme. But as so often in Iran, there was more to it.
According to British academics, Dr Mohammadi is an expert in another branch of physics, and highly unlikely to be involved in nuclear research.
At the same time, the reformist movement issued what it said was evidence that Dr Mohammadi supported their presidential candidate Mir-Hossein Mousavi in last year's election.
It all added to suspicions created by the unusually prompt and thorough coverage of his death in the Iranian media.
Whatever actually happened, the opposition will certainly fear this killing will be used as an opportunity for a new crackdown.
Another report said Dr Mohammadi had been "martyred this morning in a terrorist act by anti-revolutionary and arrogant powers' elements" - an apparent reference to Western countries.
An Iranian foreign ministry spokesman said that "signs of the triangle of wickedness by the Zionist regime, America and their hired agents, are visible in the terrorist act".
Reports also described Dr Mohammadi as a nuclear physicist, but it later appeared that his field of study was quantum theory.
There was also confusion as to whether the attack had any political overtones.
One university official said Dr 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.
Tensions have been high in Iran since the disputed election led to mass protests against the government.
The BBC's Tehran correspondent Jon Leyne, who is in London, says the opposition will fear that the blast will be used against it as part of a crackdown, our correspondent adds.
No group has said it carried out the attack.
There has been much controversy over Iran's nuclear activities.
Tehran says its nuclear programme is for peaceful energy purposes, but the US and other Western nations suspect it of seeking to build nuclear weapons.