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The world's deadliest assassins

The assassination was clinical. Tehran's city streets were gridlocked with morning rush-hour traffic as the motorcycle zipped effortlessly through the maze of vehicles. Spotting its target, it momentarily hovered alongside the silver Peugeot 405 before speeding off into the jungle of traffic.

A few seconds later, the bomb stuck to the Peugeot's window detonated, instantly killing one man inside and wounding his wife and driver.

Several kilometres to the west of the city, an identical hit was in progress. However, as the motorcycle sped away from the side of the vehicle, the vigilant driver noticed the device attached to the window. He grabbed his wife and jumped from the car before the bomb exploded. They were left bloodied, but alive.

The dead man was Majid Shahriari, a senior Iranian nuclear scientist who was, according to the head of Iran's nuclear programme, "in charge of one of the great projects" at Iran's atomic energy agency.

The wounded man who escaped with his life from the second vehicle was Fereydoun Abbasi, a 52-year-old nuclear scientist and a key member of the team running Iran's secret nuclear weapons programme.

The attacks were almost identical to the murder in January of Masoud Amimohammadi, a particle physicist who had links with Iran's nuclear programme. He was killed on his way to work in rush-hour traffic in Tehran by a bomb strapped to a motorcycle.

The message seems clear. Someone is set on killing any nuclear scientist linked with Iran's nuclear programme. And in Tehran, there are two words on everyone's lips when it comes to the likely perpetrators -- the Mossad.

The attacks have all the hallmarks of Israel's secret service, which has carried out many such assignations in foreign countries over the decades.

The Institute for Intelligence and Special Operations ('Mossad' in Hebrew simply means 'the institute') was founded in 1951 and has an estimated staff of just 1,200. This makes it one of the world's smallest secret service agencies, but its strength lies in its network of tens of thousands of carefully selected 'sayanim' ('helpers' in Hebrew) who provide safe houses, finance, cars or whatever is required by agents.

Their routine is well rehearsed and typically involves a team of undercover agents monitoring the target for months to assess routines and vulnerabilities. After patient and intricate planning, once the target is eliminated the agents are usually on their way out of the country by the time local police arrive on the scene.

Mossad operatives are far removed from the stereotypical James Bond secret agent. They are chosen to blend in and look unassuming. An agent is more likely to have a beer belly and a comb-over than bulging biceps and perfect pecs.

Mossad's covert work has been a thorn in the side of Iran's nuclear programme since its inception. It is believed to be behind several attacks on nuclear scientists in Iran, including Ardeshire Hassanpour, who died mysteriously from 'gas poisoning' in 2007.

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In September, the Stuxnet virus, which is believed to be an Israeli creation, infected the computers of Iran's Bushehr nuclear power station, crippling its IT systems and delaying its opening by two months, to January.

Fear of Mossad has already seen several Iranian nuclear scientists defect, including Shahram Amiri who disappeared last year in mysterious circumstances. He turned up in the US in March of this year under the protection of the CIA.

The two latest hits in Tehran were seen as the last hurrah of Meir Dagan, the Mossad chief who just retired after eight years at the helm of one of the world's most feared secret services.

When he was appointed in 2002, the then Prime Minister Ariel Sharon ordered him to run the agency "with a knife between its teeth". He took the order to heart and often threw diplomatic caution to the wind in order to successfully complete missions.

However, the assassination in Dubai earlier this year of Mahmoud al-Mabhouh, who served as Hamas liaison with Iran, caused intense political fallout. Local police were able to identify alleged members of a large Mossad hit team, while the use of foreign -- including Irish -- passports resulted in extreme embarrassment for Israel. It's thought to be one of the key reasons Mr Dagan was not asked to stay on.

His replacement Tamir Pardo, a long-time Mossad veteran, is a self-confessed lover of Irish traditional music and frequently holidays in Ireland. But insiders expect his tenure to be as ruthless as his predecessor's.

Wikileaks quoted Mr Dagan as telling a US senator in 2005, "Iran has decided to go nuclear ... and nothing will stop it." Another leaked cable highlights a meeting in 2007 at which Mr Dagan presented the Americans with a strategy for toppling the Iranian regime. His plan was to use minimal military force but unspecified "covert measures" while igniting unrest among Iranian minorities and exploiting the country's economic distress to undermine the regime.

Wikileaks cables suggest the Israeli government now believes small operations are no longer enough, and that defeating Iran will require the use of military force. But for now the front line in Israel's attempt to thwart Iran's nuclear ambitions will remain hidden in the murky shadows of cloak-and-dagger activities. The worrying thing for Iran's nuclear scientists is that Majid Shahriari is unlikely to be the last to cross the path of Mossad.

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