An Iranian physicist was gunned down yesterday near his home in south Tehran, according to Iranian media reports.

According to the reports, based on police sources, Darioush Rezaei, 35, was shot dead by two gunmen firing from motorcycles. Rezaei's wife was injured in the attack and rushed to hospital. This is the fourth attack on an Iranian nuclear scientist in the past year. In the previous cases, Iranian media outlets and spokesmen accused the Mossad, the CIA and MI6 of being behind the strikes.

Rezaei, who was a university lecturer in the city of Ardabil, did his doctorate in neutron transport - which lies at the heart of nuclear chain reactions in reactors and bombs - at the Ferdowsi University of Mashhad.

He was a member of the Atomic Energy Organization of Iran, the country's official atomic energy commission. Ostensibly this agency is in charge of its civilian nuclear program, but according to various reports it is also involved, together with secret groups in the Revolutionary Guards, in Iran's nuclear weapons' production.

As of last night, official Iranian spokesmen had not issued a response to the killing.

In November 2010, another scientist, Majid Shahriari, was killed and on the same day an attempt was made on the life of nuclear laser expert Prof. Fereidoun Abbasi, injuring him.

Abbasi was later appointed head of the Atomic Energy Organization of Iran. Two months ago, he attended a meeting in Vienna of the International Atomic Energy Agency, where he made clear that Iran would continue its nuclear program and would not be deterred by pressure.

Earlier in 2010, Prof. Massoud Ali Mohammadi was killed in Tehran by a bomb attached to his car.

According to Iran, the attacks show that those responsible - Iran blames Israel and the Mossad - are determined to strike at scientists involved in Iran's nuclear program.

All of the targeted Iranian nuclear scientists taught and conducted research at university physics departments. But according to Western officials, they were also working secretly for Iran's military nuclear program.

The attacks seem to be focused on taking out key people involved in the last and most important step on the road to nuclear weapons - the group known as the weapons group.

This is the stage at which the bomb is assembled, mainly an engineering process by which the fissionable materials are inserted into the bomb and an explosive mechanism created.

Iran is believed to have all the knowledge, ability and technology to manufacture a nuclear weapon but has still not assembled the bomb and is even farther from assembling a nuclear warhead on a missile.

To produce weapons-grade explosives, Iran needs to enrich the uranium it already has to 93 percent. This process would take six months from the moment a decision is made to do so, although it cannot be ruled out that Iran has already begun the process at a small facility unknown to intelligence agencies.