The attack - which instantly killed the scientist and fatally wounded his driver on Wednesday - was at least the fourth targeted hit against a member of Iran's nuclear brain trust in two years. Tehran quickly blamed Israeli-linked agents backed by the U.S. and Britain.
Secretary of State Hillary Rodham Clinton denied any U.S. role in the slaying, and the Obama administration condemned the attack. However, provocative hints from Jerusalem reinforced the perception of an organized and clandestine campaign to set back Iran's nuclear ambitions.
The day before the attack, Israeli military chief Lt. Gen. Benny Gantz was quoted as telling a parliamentary panel that 2012 would be a "critical year" for Iran - in part because of "things that happen to it unnaturally."
The blast killed Mostafa Ahmadi Roshan, a chemistry expert and a director of the Natanz uranium enrichment facility, the centerpiece of Iran's expanding program to make nuclear fuel. Roshan, 32, had planned to attend a memorial later Wednesday for another nuclear researcher who was killed in a similar pinpoint blast two years ago, Iranian media said.
"A heinous act," said Iran's Atomic Energy Organization of Wednesday's bombing.
It added a tone of defiance. "We will continue our (nuclear) path without any doubt ... Our path is irreversible," said the statement carried on state television.
The state news agency IRNA said Roshan had "organizational links" to Iran's nuclear agency, which suggests a direct role in key aspects of the program. Another news agency, the semiofficial Mehr, said Roshan had been interviewed by inspectors from the U.N. nuclear watchdog agency - which Iran has accused of placing its scientists in peril by including their names in public reports.
Natanz, in central Iran, is the country's main enrichment site. Officials said this week they were expanding some operations to an underground site south of Tehran with more advanced equipment.
The U.S. and its allies are pressuring Iran to halt uranium enrichment, a key element of the nuclear program that the West suspects is aimed at producing atomic weapons. Uranium enriched to low levels can be used as nuclear fuel, but at higher levels it can be used as material for a nuclear warhead.
Iran denies it is trying to make nuclear weapons, saying its program is for peaceful purposes only and is geared toward generating electricity and producing medical radioisotopes to treat cancer patients.
The years of virtual stalemate between Iran and the West appear to be shifting into a new period of heightened pressures and tensions.
Russia strongly warned the West on Wednesday against any attack on Iran, saying it would upset global security.
Military action would be a "grave mistake, a flagrant error" with far-reaching consequences for regional and global stability, Deputy Foreign Minister Sergei Ryabkov told the ITAR-Tass news agency. "It could shake the foundations of the international system."
Tehran has accused Israel's Mossad, the CIA and Britain's spy agency of engaging in an underground "terrorism" campaign against nuclear-related targets, including at least three other slayings since early 2010 and the release of a malicious computer virus known at Stuxnet in 2010 that disrupted controls of some centrifuges - a component in nuclear fuel production. All three countries have denied the Iranian accusations.
Speaking in Washington, Clinton strongly denied any U.S. role in the latest attack.
"I want to categorically deny any United States involvement in any kind of act of violence inside Iran," she said. "We believe there has to be an understanding between Iran, its neighbors and the international community that finds a way forward for it to end its provocative behavior, end its search for nuclear weapons and rejoin the international community and be a productive member of it."
Israeli officials, however, hinted at covert campaigns against Iran without directly admitting involvement.
"Many bad things have been happening to Iran in the recent period," said Mickey Segal, a former director of the Israeli military's Iranian intelligence department. "Iran is in a situation where pressure on it is mounting, and the latest assassination joins the pressure that the Iranian regime is facing."
Iranian authorities pointed the finger at arch-foe Israel.
First Vice President Mohammad Reza Rahimi said Israeli agents were behind the assassination, but cannot "prevent progress" in what Iran claims are peaceful nuclear efforts.
Roshan was inside the Peugeot 405 together with two others when the bomb exploded near Gol Nabi Street in north Tehran, Fars reported. It said Roshan's driver later died at a hospital from wounds sustained in the attack. An 85-year old passer-by was reportedly wounded in the blast.
Fars described the explosion as a "terrorist attack" targeting Roshan, a graduate of the prestigious Sharif University of Technology in Tehran. Police draped the bomb-ravaged car with a blue tarp and hosed blood from the pavement. Some bits of the vehicle were hurled into the bare branches of trees.
Roshan was a chemistry expert who was involved in building polymeric layers for gas separation, which is the use of various membranes to isolate gases. He was also deputy director of commercial affairs for the Natanz uranium enrichment plant in central Iran. According to conservative news website mashreghnews.ir, Roshan was in charge of purchasing and supplying equipment for the facility.
Natanz remains the mainstay of Iran's uranium labs. But Iran said this week it was expanding some operations to a bunker-like site south of Tehran protected under 300 feet (90 meters) of rock. The existence of the Fordo facility has been known for more than two years, but some Western officials fear the move could be another step toward developing nuclear arms.
The conservative news website, alef.ir, posted several papers to which Roshan contributed. It said his specialty, polymeric layers, has uses in uranium enrichment by having uranium gas pass through filtering membranes.
Since December, Iran has held or announced a series of war games that included threats to close the Gulf's vital Strait of Hormuz - the passageway for about one-sixth of the world's oil - in retaliation for stronger U.S.-led sanctions.
"Assassinations, military threats and political pressures ... The enemy insists on the tactic of creating fear to stop Iran's peaceful nuclear activities," Fars quoted lawmaker Javad Jahangirzadeh as saying in reaction to the blast.
A similar bomb explosion on Jan. 12, 2010 killed Tehran University professor Masoud Ali Mohammadi, a senior physics professor. He died when a bomb-rigged motorcycle exploded near his car as he was about to leave for work.
In November 2010, a pair of back-to-back bomb attacks in different parts of the capital killed another nuclear scientist and wounded one more.
The slain scientist, Majid Shahriari, was a member of the nuclear engineering faculty at Shahid Beheshti University in Tehran and cooperated with the Atomic Energy Organization of Iran. The wounded scientist, Fereidoun Abbasi, was almost immediately appointed head of Iran's atomic agency.
Shahriari's expertise - neutron transport - lies at the heart of nuclear chain reactions in reactors and bombs. And Abbasi, now Iran's nuclear chief, has been described as a laser expert and one of the few top Iranian specialists in nuclear isotope separation.
In July 2011, motorcycle-riding gunmen killed Darioush Rezaeinejad, an electronics student. Other reports identified him as a scientist involved in suspected Iranian attempts to make nuclear weapons.
Rezaeinejad allegedly participated in developing high-voltage switches, a key component in setting off the explosions needed to trigger a nuclear warhead.
"Instead of actually fighting a conventional war, Western powers and their allies appear to be relying on covert war tactics to try to delay and degrade Iran's nuclear advancement," said Theodore Karasik, a security expert at the Dubai-based Institute for Near East and Gulf Military Analysis.
Meir Javedanfar, an Iranian-born analyst based in Israel, said Iran's leadership is being pushed toward a decision on whether to "retaliate or compromise" as sanctions squeeze the economy and undercut the value of the Iranian rial.
"From the international consensus that we can see against Iran, even if (Supreme Leader Ayatollah Ali Khamenei) does retaliate, it's not very likely that the pressure - sanctions and isolation - would ease," he said. "He's in a tight spot."