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The two assassins arrived from nowhere as their victim was driving home with his wife. Trapped inside his car, he was hopelessly vulnerable as their motorcycles pulled alongside.

He would just have had time to notice their blacked-out visors before they opened fire, emptying round after round into his chest.

Nuclear scientist Darioush Rezaei died immediately. His wife was critically wounded and still in hospital days after the attack in north eastern Iran. The hitmen? They vanished into the traffic fumes of the night.

This is a story of ruthless men playing for the highest stakes imaginable. Of secret agents from Israel's intelligence service Mossad who will stop at nothing to prevent Iran from acquiring a nuclear weapon.

Should Iran succeed, Israel would be desperately vulnerable to attack - not least because Iran's President Ahmadinejad has repeatedly threatened to erase the 'Zionist entity' from the map.

Comment: The only thing on repeat here is the media's regurgitation of a false translation of what Ahmadinejad actually said:

Farsi: "Imam ghoft een rezhim-e ishghalgar-e qods bayad az safheh-ye ruzgar mahv shavad."

English: "The Imam said this regime occupying Jerusalem must vanish from the page of time".

There's also the danger of nuclear proliferation among Israel's Arab neighbours. If Ahmadinejad gets hold of a nuclear weapon, Egypt, Saudi Arabia and others would immediately seek to do so as well, to prevent Iran from bullying them with its new-found power. Israel's response to the threat has been deadly.

Rezaei was assassinated because he was an expert on neutron transport, one of the key processes in making nuclear weapons. He joins a long list of Iranian nuclear scientists and engineers who are being systematically targeted by killers apparently dispatched by the Israeli intelligence agency. While it is unlikely Mossad would send its own assassins into such a high-risk environment, they will have recruited locals and given them intensive training.

Last November, two senior Iranian scientists were attacked in different parts of the capital. Both victims were driving to work when men on motorbikes attached magnetised bombs to their cars as they were stuck in traffic.

These small explosives are known as 'shaped charges', designed to focus the blast at its target as a stream of molten metal travelling at 29,000 miles per hour. One bomb killed nuclear engineer Majid Shahriari, while missing his wife in the passenger seat.

In another part of town, nuclear engineer Dr Fereydoon Abbasi narrowly survived an identical attack. Dr Abbasi is an expert in the separation of isotopes, a crucial process in the manufacture of enriched uranium fuel, which has uses in both nuclear reactors and weapons.

In January, it was the turn of 50-year-old Masoud Ali-Mohammadi, who was killed near his north Tehran home by a remotely detonated bomb built into a motorcycle parked on the route he took to work each morning. The bomb blew Mohammadi's car to pieces.

Although his Western scientific colleagues claim that the dead man was an expert in quantum mechanics rather than nuclear fission, it has since emerged that for 20 years he was a member of Iran's Revolutionary Guard, the key government agency involved in developing Iran's nuclear weapons.

The deaths follow a pattern that can be traced back to 2007, when Dr Ardeshir Hosseinpour, a scientist employed at the top-secret Istfahan nuclear plant, mysteriously died of radiation poisoning. Of course, Israel denies any connection with these deaths. But intelligence experts are convinced Mossad is behind them, sometimes carrying out the killings in conjunction with like-minded intelligence agencies, including the CIA.

For the past five years, the CIA's Project Brain Drain has been trying to lure jobless Iranian science graduates to the US in order to denude Iran of potential nuclear bomb makers. The CIA has also tried to entice the country's more senior nuclear scientists to defect - but only half a dozen have done so.

Israel has never made a secret of its policy that those who harm it will be harmed in turn. Yet the killing of Iranian engineers and scientists in increasing numbers smacks of desperation.

The Israelis had hoped to persuade America to help them attack nuclear facilities in Iran, which are buried deep underground. But the US does not wish to get involved in another war, and refused to supply Israel with high-tech bunker-penetrating bombs.

So, for now, Israel decided to delay Iran's research programme using sabotage and the assassination of key scientific players.