At first, Iranian officials played down Thursday's incident at the Natanz nuclear facility. It was just a fire in a shed, they said: nobody was hurt, no significant damage was done.

But video footage showing a building with its roof blown off and a door blasted from its hinges told a different story. And then came a more plausible explanation: someone had smuggled a bomb into the building.

Responsibility has been claimed by a dissidentA person who opposes official policy, especially that of an authoritarian state. Iranian group called "Cheetahs of the Homeland". But experts think it far more likely to be the work of Israel's secret service, Mossad - possibly with the help of the US.

Natanz, with laboratories built 25-feet underground to withstand air attacks, is Iran's centre for enriching uraniumA very heavy, silvery-grey metal that is naturally radioactive., which might eventually be used to create a nuclear bomb. And no country has more to fear from that than Israel.

Iran's supreme leader, Ayatollah Ali Khameini, has described Israel as "a cancerous tumour" that should be removed from the region. His regime is accused of assisting Israel's enemies HamasA militant Palestinian organisation, which has controlled the Gaza Strip since 2007. It is listed as a terrorist organisation in the UK. and HezbollahA militant organisation based in Lebanon, classified in the UK as a terrorist group. It has made direct attacks on Israel and is believed to have been responsible for assassinating Lebanon's former Prime Minister Rafic Hariri., and organising terrorist attacks on its embassies. Iran even issued a stamp commemorating an Egyptian soldier who murdered seven Israeli tourists.

In recent years, Israel has carried out a series of covert attacks on Iran's nuclear programme. Several leading scientists have been assassinated using bombs attached to cars or motorcycles.

In 2018, Mossad agents broke into a warehouse in Tehran and stole half a tonne of secret documents revealing the whereabouts of secret equipment and nuclear material.

Mossad's reputation for daring and persistence spans decades. In 1960, a team traced and kidnapped one of the architects of the Holocaust, Adolf EichmannThe senior Nazi in charge of organising the deportation of Jews to extermination camps during World War Two. He was convicted of crimes against humanity and hanged in 1961., in Argentina and smuggled him back to Israel to face trial.

Following the Munich massacreEight members of Black September killed the athletes and a German policeman during the 1972 Olympic Games, after a failed rescue attempt. in 1972, when the Palestinian Black September group murdered 11 Israeli athletes, Mossad spent 20 years tracking down the men it believed responsible and killed several of them.

In 1976, when Palestinian hijackers took over a flight from Tel Aviv and forced the pilots to land in Uganda, Mossad played a crucial role in rescuing the passengers and crew, obtaining plans of Entebbe AirportThe hijackers had the support of the Ugandan dictator Idi Amin. The Israeli commandos flew more than 2,000 miles to carry out the rescue operation. and sending an agent to take pictures of it in broad daylight. In the ensuing raid by Israeli commandos, 102 of the 106 hostages were brought out alive.

Mossad can also draw on some of the most advanced security technology in existence, thanks to a policy of investing in high-tech companies. Natanz had previously been the target of a highly sophisticated cyber attack, in which a computer virus called Stuxnet was successfully used to disrupt uranium enrichment.

Some say, yes, Mossad has a record of successes second to none. It is brilliant at training its agents and, because Israel has been involved in or threatened by armed conflict since its creation, its secret service is under constant pressure to produce results. It is also completely ruthless. As Israel's foreign minister, Gabi Ashkenazi, said on Sunday, "We take actions that are better left unsaid."

Others argue that there are equally effective secret services elsewhere: just look at the murder of
Kim Jong-un's brother by North Korean agents in Malaysia in 2017. And some Mossad operations have gone badly wrong. Six agents were arrested in Norway after killing a Moroccan waiter they mistook for a Black September leader; two were arrested in New Zealand for trying to obtain passports by fraud.