By FARNAZ FASSIHI And JAY SOLOMON
An Iranian scientist working for a key nuclear site was assassinated in Tehran with a magnetic bomb attached to his car, in what the government said was a plot by the U.S. and Israel at a time of growing strains over Iran’s nuclear program.
The U.S. and Britain both condemned the death of 32-year-old chemical engineer Mostafa Ahmadi Roshan and denied any role in what has been a series of assassinations of nuclear scientists in Iran.
A spokesman for Israeli Prime Minister Benjamin Netanyahu declined to comment on Iran’s accusation on Wednesday. Many intelligence officials and diplomats in Washington say they believe Israel has played a central role in a series of attacks on Iranians scientists.
Mr. Roshan was the fourth Iranian nuclear scientist killed in two years, a period in which the West has intensified covert actions against Tehran’s nuclear program, according to U.S. officials. Incidents including computer viruses and explosions have afflicted Iran’s nuclear program and security infrastructure.
“The United States played no role whatsoever in the killing of this scientist,” Pentagon spokesman George Little said. “We have been very clear that we seek to lower the temperature on tensions with Iran, and we think that things have calmed down a bit in recent days.”
Tensions have escalated this month after the U.S. and European Union moved to impose tougher sanctions on Iran, including a proposed EU embargo on oil imports and a U.S. ban on working with Iran’s central bank, the backbone of its economy.
Iran is doing its part to fuel the fire. On Monday, an Iranian court handed a death sentence to a 28-year-old Iranian-American ex-Marine, Amir Hekmati, after convicting him of spying for the Central Intelligence Agency. Iranian officials have threatened to block the Strait of Hormuz, a key oil route, prompting U.S. assertions that it wouldn’t let that happen.
Iranian officials blamed the U.S. and Israel for the killing of Mr. Roshan and said it was a plot to create unrest before a parliamentary election in March and because the West had reached a dead end in nuclear negotiations with Iran.
Iranian nuclear scientists have been the target of assassinations for over two years.
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“Those who claim they are fighting terrorism today are targeting Iranian scientists but they should know that these terrorist actions only strengthen our resolve to achieve the Islamic Republic’s ideals,” said Vice President Mohammad-Reza Rahimi.
Iran’s ambassador to the U.N., Mohammad Khazaee, on Wednesday night called for the Security Council to condemn the attack, adding that “any kind of political and economic pressures or terrorist attacks targeting the Iranian nuclear scientists could not prevent our nation in exercising” its right to use nuclear energy for peaceful purposes.
While Mr. Netanyahu’s government has repeatedly declined to confirm or deny involvement in the assassinations of Iranian scientists, Israeli officials have said they have drawn some clear red lines that, if crossed, would lead Israel to take action against Tehran.
One of those lines was breached over the weekend when the head of Iran’s Atomic Energy Organization, Fereydoon Davani-Abbasi, said Tehran had begun enriching uranium at an underground nuclear facility near the holy city of Qom.
Mr. Davani-Abbasi said the enrichment plant, called Fordow, would process enriched uranium to a 20% purity level, moving Iran dangerously close to producing weapons-grade fuel. The facility is buried in a mountain complex surrounded by antiaircraft batteries and viewed largely immune from an attack.
Israeli officials have said they are working to deny Iran a “sphere of immunity” from military action. Western countries allege that the country is pursuing development of nuclear weapons. Iran says its nuclear program is peaceful.
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In briefings with U.S. lawmakers on Capitol Hill, American military and intelligence officials have cited the killing of the Iranian scientists as one of three key factors that have delayed the Iranian nuclear program. The other two, they said, were the so-called Stuxnet computer virus and Iran’s own technical shortcomings.
It is unlikely that Israel would involve the U.S. or even warn of a hit, according to former U.S. officials. Israel wouldn’t need the U.S. for intelligence-gathering or planning for an attack, they said, and they would have little incentive to notify the U.S. beforehand because in the past the U.S. has lectured on the morality of such operations.
Mr. Roshan was a chemical engineer and the deputy director of commerce for the Natanz uranium enrichment facility, where he was responsible for purchasing equipment for the facility, according to the IRNA state-news agency.
Diplomats and analysts said Mr. Roshan’s believed role as a senior procurement officer at Natanz likely was the reason he was targeted. Such a job would place him at the center of Iran’s efforts to procure materials and equipment for the production of nuclear fuel. He may have also played a role in trying to develop defense at Natanz to combat sabotage operations, analysts said.
“This guy sounds like he had multiple jobs. But the one that’s most important is that he handled procurement,” said Paul Brannan of Washington’s Institute for Science and International Security, which tracks Iran’s nuclear program. “Someone who’s handling this is dealing with sabotage and other efforts to stall the program.”
Iran is a police state with a highly capable intelligence and military apparatus that would make it difficult for amateurs to carry out attacks such as the one that killed Mr. Roshan.
“It is indeed the Israelis who have the capability to do something like that, it’s very hard to do something like this on foreign territory, or it could be an internal job,” said Joseph Wippl, a professor at Boston University and a former CIA officer.
Among attacks in the past two years, in 2010, Mr. Davani-Abbasi, the Iranian atomic chief, narrowly survived a bombing that was staged nearly simultaneously with an attack that killed another nuclear scientist. Both attacks were made with magnetic bombs attached to cars.
The attacks are controversial in the West. Some U.S. and European officials said they could intimidate Iranian scientists from participating in Iran’s nuclear program. But they said the actions could produce sympathy for Iran in the international community and undercut some countries’ willingness to cooperate in squeezing Iran.
Experts on Iran’s nuclear program also said they questioned just how much it would be damaged by the deaths of a few scientists.
The attacks also risk hurting U.S. interests. The death of the Iranian scientist could raise the stakes for Mr. Hekmati, the ex-Marine who faces a death sentence in Iran. His attorney has less than three weeks to appeal the sentence. The U.S. maintains the charges were fabricated and part of a pattern by Iran of arresting innocent people for political reasons.
—Siobhan Gorman and Joshua Mitnick
contributed to this article.