From BBC       12 January 2010

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.

Mr Ali Mohammadi, 50, was killed by a remote-controlled bomb near his home.

'Martyred'

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.

Another report said Mr 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 Mr 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 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.

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.