Neither Israeli nor American officials will discuss the covert campaign in any detail, leaving some uncertainty about the perpetrators and their purpose. For instance, Karim Sadjadpour, an Iran expert at the Carnegie Endowment for International Peace, said he believed that at least some of the murdered scientists might have been killed by the Iranian government. Some of them had shown sympathy for the Iranian opposition, he said, and not all appeared to have been high-ranking experts.

“I think there is reason to doubt the idea that all the hits have been carried out by Israel,” Mr. Sadjadpour said. “It’s very puzzling that Iranian nuclear scientists, whose movements are likely carefully monitored by the state, can be executed in broad daylight, sometimes in rush-hour traffic, and their culprits never found.”

A more common view, however, is expressed by Patrick Clawson, director of the Iran Security Initiative at the Washington Institute for Near East Policy. “I often get asked when Israel might attack Iran,” Mr. Clawson said. “I say, ‘Two years ago.’ ”

Mr. Clawson said the covert campaign was far preferable to overt airstrikes by Israel or the United States on suspected Iranian nuclear sites. “Sabotage and assassination is the way to go, if you can do it,” he said. “It doesn’t provoke a nationalist reaction in Iran, which could strengthen the regime. And it allows Iran to climb down if it decides the cost of pursuing a nuclear weapon is too high.”

A former senior Israeli security official, who would speak of the covert campaign only in general terms and on the condition of anonymity, said the uncertainty about who was responsible was useful. “It’s not enough to guess,” he said. “You can’t prove it, so you can’t retaliate. When it’s very, very clear who’s behind an attack, the world behaves differently.”

The former Israeli official noted that Iran carried out many assassinations of enemies, mostly Iranian opposition figures, during the 1980s and 1990s, and had been recently accused of plotting to kill the Saudi ambassador to the United States in Washington.