Amid escalating threats, the covert war to thwart Iran's efforts to get nuclear weapons took an ugly – if gruesomely familiar – turn Wednesday with the murder of a young Iranian nuclear scientist on a Tehran street.
It was the fourth such reported targeted assassination in two years, adding a dangerous new element to the escalating conflict over Iran's refusal to rein in its nuclear program or to open it to international inspection.
Wednesday's killing in North Tehran was similar to previous attacks. Using powerful magnets, a motorcyclist attached a small delayed-action bomb to a car carrying Mostafa Ahmadi-Roshan, a nuclear scientist and university professor.
The explosion killed the 32-year-old chemistry professor, who worked at the sprawling Natanz nuclear facility, and another person in the car, reports said. The pinpoint attack focused the blast into the car during the morning rush hour.
Despite his relative youth, some Iranian reports said the professor was a senior official at the Natanz uranium enrichment facility. Iran's Vice-President Mohammad Reza Rahimi said there was "evidence of [foreign] government-sponsored terrorism."
There were similar targeted killings of Iranian nuclear scientists in January and November of 2010, and bombs have injured others.
The United States and Iran have ratcheted up hostilities in recent weeks, accusing each other of being sponsors of state terrorism as Tehran has threatened to close the vital sea lanes of the Straits of Hormuz, while Western nations have urged tough new sanctions to curb Iran's nuclear ambitions.
After Wednesday's attack, Tehran accused Israel and the United States of staging what it called the "heinous" killing. It vowed to continue developing its nuclear program.
"Assassinations, military threats and political pressures ... the enemy insists on the tactic of creating fear to stop Iran's peaceful nuclear activities," said Javad Jahangirzadeh, a member of Iran's parliament, according to the semi-official Fars news agency.
Although Israel and the United States routinely deny any involvement in the targeted assassinations of key Iranian nuclear personnel, both countries regard Tehran's pursuit of nuclear weapons as a threat.
U.S. President Barack Obama has vowed to prevent Tehran from joining the world's small nuclear-armed club, making clear that military force will, if needed, be used against Iran's widely dispersed and often-buried nuclear sites.
The American administration officially denied it had a hand in the latest assassination.
"I want to categorically deny any United States involvement in any kind of act of violence inside Iran," U.S. Secretary of State Hillary Clinton said, adding that Iran must "end its provocative behaviour [and] end its search for nuclear weapons."
With the U.S. Fifth Fleet maintaining a powerful and highly-evident presence in the Persian Gulf – twice in the last few days, American warships have rescued Iranian sailors, once freeing them from pirates – the risk of a maritime confrontation is also rising.
Russia, Iran's main nuclear-technology supplier, expressed fears of further escalation, and warned that military action could spark a regional war in the oil-rich Gulf. It would a "grave mistake, a flagrant error" with far-reaching consequences for regional and global stability, Deputy Foreign Minister Sergei Ryabkov said in Moscow.
Since the downing of a sophisticated American spy drone last month, Tehran and Washington have swapped accusations, escalating a simmering hostility that stretches back decades to the 1979 Islamic revolution that toppled the U.S.-installed Shah of Iran.
Last fall, Washington accused Tehran's Quds network of funding a terrorist plot to kill the Saudi ambassador. This week an Iranian judge delivered a death penalty verdict on a U.S. citizen, a former military translator considered a CIA spy. Washington says that 28-year-old Amir Hekmati was just visiting his Iranian grandmothers.
Meanwhile, both the huge and powerful U.S. Navy and Tehran's tiny but bold fleet of pesky, missile-firing launches have been "showing the flag" in the Persian Gulf.
Mr. Obama recently slapped Tehran with a new round of sanctions. America's European allies are mulling an embargo on buying Iranian oil and the latest UN nuclear watchdog report raises grave doubts about Iran's claims that its nuclear program is solely for generating electricity.
With his Republican rivals vying to be the most bellicose on Iran, Mr. Obama also has stepped up the tough talk in recent weeks.
Iran insists its nuclear program is purely for peaceful, power-generating purposes, but also boasts of a new enrichment effort achieving 20-per-cent levels. Although that's still below weapons-grade, outside analysts fear it's just a step to nuclear-weapons levels of enrichment. It has long hidden aspects of the program and continues to defy international inspections.
Israel, along with India, Pakistan and most recently North Korea are the nations known to have nuclear weapons in defiance of an international treaty limiting their legal possession to Britain, France, China, Russia and the United States.
The covert efforts to disrupt Iran's nuclear program range far beyond the killing of scientists. There have been mysterious software attacks using the Stuxnet virus, which crippled thousands of centrifuges spinning Iranian uranium to weapons-purity levels.
Those cyber attacks are also believed to be part of a clandestine U.S.-Israeli campaign to derail Iran's nuclear-weapons efforts. In Washington, were such attacks directed at U.S. installations, they would be regarded as an act of war.