Israel and US behind Tehran blast - Iranian state media
BBC Persian's Sadeq Saba says the attack is surrounded by mystery
Iranian state media have accused Israel and the US of being involved in a bomb attack which killed an Iranian physicist in Tehran.
State broadcaster Irib quoted Iran's foreign ministry spokesman as saying there were signs of Israeli and US involvement "in the terrorist act".
The US state department dismissed the allegation as "absurd".
Masoud Ali Mohammadi - described as a "devoted revolutionary professor" - was killed by a remotely-controlled bomb.
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, Mr 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 Mr 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.
Reports in the Iranian media described Mr Mohammadi as a nuclear physicist, but it appears 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 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.
Tensions have been high in Iran since the disputed election led to mass protests against the government.
Mr Mohammadi, who worked at Tehran University, "was killed in a booby-trapped motorbike blast" in the city's northern Qeytariyeh district, state-run Press TV reported earlier.
It showed pictures from the scene of the blast, saying windows in the nearby buildings had been shattered by the force of the explosion.
'Triangle of wickedness'
Local media reports say the bomb was attached to a motorcycle parked outside Mr Mohammadi's home, although one agency said it had been planted in a rubbish bin.
Professor Mohammadi was killed as he was leaving his home, media say
Irib later quoted Iran's foreign ministry spokesman as saying that "in the initial investigation, signs of the triangle of wickedness by the Zionist regime, America and their hired agents, are visible in the terrorist act".
Press TV quoted security officials at the scene as saying that 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.
In its earlier report, Irib said Mr Mohammadi "was martyred this morning in a terrorist act by anti-revolutionary and arrogant powers' elements".
The BBC's Tehran correspondent Jon Leyne, who is in London, says Iran usually refers to its enemies in the West as "the arrogant powers".
The opposition in Iran will fear that Tuesday's blast will be used against it as part of a crackdown, our correspondent adds.
Police sealed off the area and launched an investigation into the incident.
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.
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.
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.
In December, Tehran accused Saudi Arabia of detaining an Iranian nuclear scientist and handing him over to the US.
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