Evidence of a covert war against Iran's nuclear program is mounting. Yesterday's assassination of an Iranian scientist tied to the program is just the most recent data point in the last year that indicates an accelerating effort to spread fear and slow the country's nuclear work.

Mostafa Ahmadi Roshan, a chemist who the Iranian state press says was the marketing director for Iran's Natanz nuclear facility, was the latest victim. He was killed by a small bomb affixed to the underside of his car, the same method used to murder nuclear physicist Massoud Ali Mohammadi in Tehran exactly a year ago. In all, four scientists connected to nuclear work in Iran have been killed since the start of 2010.

In September 2010, the Stuxnet computer virus, the most sophisticated cyberweapon deployed in history, was uncovered. The virus – which computer security efforts said was too complex to have been built without a large team and extensive resources – targeted Iranian nuclear enrichment centrifuges designed to produce highly enriched uranium, slowing Iran's enrichment for months.

Though Iran says its nuclear program is simply meant to produce power, highly enriched uranium could also be used in the production of a nuclear bomb – the aspect of the program that US and Israeli officials find most worrying.

Whoever is responsible, the murders appear to be as much about spreading terror as they are about stopping the nuclear program. Iran has legions of capable engineers, and none of the victims appear to have had indispensable knowledge or abilities. According to the Congressional Research Service, Iran's main nuclear research complex in Isfahan probably has 3,000 employees alone, and the there are about 10 other major nuclear sites in the country.

But spreading fear among the living can slow them down, spread confusion, or deter young recruits. If scientists became frightened enough, they might be reluctant to travel to work or conferences inside the country. Meanwhile, enhanced security measures at the sites could prove cumbersome.

In November 2010, Majid Shahriari, a nuclear engineer at Shahid Beheshti University in Tehran, was killed by a car bomb. In July 2011, engineer Darioush Rezaeinejad, believed to be working within Iran's secretive nuclear program, was gunned down in the street. Last November, Maj. Gen. Hassan Moghaddam, considered the father of Iran's missile program, died with 16 others in a still unexplained explosion at a military base.

If Ian Fleming's super-villain Auric Goldfinger, the antagonist in the Bond book of the same name, was right to say once is coincidence, twice is happenstance, and three times is enemy action, then there should be little doubt that a coordinated assassination campaign has been under way in Iran for some time. Suspicion has turned to Israel, since it's the nation most alarmed by Iran's nuclear program and has carried out assassination efforts abroad in the past. After the 2010 murder of a senior Hamas official in Dubai, strong circumstantial evidence pointed to the Mossad, Israel's secret intelligence service.

After Gen. Moghaddam's death, Israeli Intelligence Minister Dan Meridor told Israel's Army Radio that "not every explosion over there should be tied to espionage and stories from the movies," though he went on to imply that Israel was willing to use violence over Iran's nuclear program. "There are countries who impose economic sanctions and there are countries who act in other ways," Mr. Meridor said then.

Israeli officials have been studiously ambiguous in their comments on the murders in Iran. That would make sense if Israel was responsible – and if it wasn't. After all, if the murders are being carried out by someone else (perhaps the US, though the White House says the US was not involved, or perhaps as part of some internal Iranian rivalry), it doesn't hurt to get some of the credit, particularly if it has the consequence of creating more fear and doubt in an enemy state.

"There are other messages in these campaigns: one is to terrorize those who are working in [the nuclear program] already. The second is targeted to young scientists thinking of joining," Haaretz columnist Yossi Melman told the Monitor's Joshua Mitnick for a story we published this morning. "The third message is to the regime and population: The message is, 'We can get you anywhere, any time.' The regime is seen as weak."

Is any of this having the desired effect? Iran has continued to insist that it's nuclear work will stream ahead and that it won't be cowed.

In a letter to the United Nations Security Council yesterday, Iran's UN Ambassador Mohammad Khazaee asked for the international community "to condemn, in the strongest terms, these inhumane terrorist acts" and said Iran will not be deterred. "Any kind of political and economic pressures or terrorist attacks targeting the Iranian nuclear scientists, could not prevent our nation in exercising this right" to nuclear program, he wrote.

But Iran is increasingly isolated, and the odds for international support over the killings are slim. As Reuters pointed out yesterday, the UN General Assembly voted overwhelmingly to condemn an alleged Iranian plot to kill Saudi Arabia's ambassador to the US last year, but has not taken action to condemn the murders of Iran's scientists.

Analysts have speculated that centrifuge problems at Natanz were caused by Stuxnet and slowed down production rates for months. As for the killings, there has been no evidence yet of scientists abandoning the nuclear program out of fear, or of the loss of a key member of the Iranian team.

US Defense Secretary Leon Panetta said over the weekend that sanctions and international pressure against Iran's nuclear program are "working," and indeed, crushing financial sanctions are probably doing more to complicate Iran's nuclear work than any covert efforts from abroad. Asked about Iran's intentions, Mr. Panetta said, "Are they trying to develop a nuclear weapon? No. But we know that they're trying to develop a nuclear capability and that's what concerns us and our red line to Iran is: Do not develop a nuclear weapon."

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By "capability" he's referring to Iran developing all the know-how and material needed for a bomb, without taking the final step towards assembly, perhaps keeping that in reserve for a moment when they felt threatened enough – or secure enough – to go the final few yards.

But fear can be a double-edged sword. Were Iran to successfully build a bomb, the nature of the whole game would be changed. Efforts to stop its nuclear work would of necessity shift towards finding ways to live with a new nuclear power – just as the world had to learn to live with first a nuclear China, then later a nuclear Israel, India, and Pakistan.