Iranian investigators gather evidence at the site of the blast near a university in northern Tehran.
Iran says the assassination of one of its nuclear scientists in Tehran will not stop the progress of its nuclear program, in the fourth attack targeting an Iranian nuclear physicist in two years.
Calling the assassination "evidence of [foreign] government-sponsored terrorism," Vice President Mohammad Reza Rahimi told state television that the attack made Iran's "scientists more determined than ever in striding toward Iran's progress."
Two men on a motorcycle reportedly attached a magnetic bomb to the car of nuclear scientist Mostafa Ahmadi Roshan as he drove in northern Tehran early on January 11, killing him when the bomb exploded moments later.
A woman told Iranian state television: "I saw a motorcycle. They were wearing woolen hats -- black woolen hats. [There were] two people. I saw the motorcycle speed by. I saw them. It appeared as though they had something in their hands."
Roshan's driver/bodyguard later died of his wounds, while a third occupant of the car remains in hospital.
Iran's Atomic Energy Organization released a statement confirming Roshan was a nuclear scientist and calling his killing a "heinous act."
Fars news agency said that Roshan, aged 32, supervised a department at Iran's uranium-enrichment facility in Natanz, near Isfahan.
Tehran's deputy governor blamed the assassination on Israel. Safar Ali Baratloo told media that "the bomb was a magnetic one and the same as the ones previously used for the assassination of [other] scientists and is the work of Zionists."
WATCH: The aftermath of the bomb attack in Tehran
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Iranian Nuclear Scientist Killed In Bomb Attack
Tehran has frequently accused both Israel and the United States of killing nuclear scientists to weaken its nuclear program.
In Washington, U.S. Secretary of State Hillary Clinton "categorically" denied "any United States involvement in any kind of act of violence inside Iran."
White House spokesman Tommy Vietor also condemned Roshan's death and said the United States "had absolutely nothing to do" with it.
In a message posted on Facebook, Israel's chief military spokesman, Brigadier General Yoav Mordechai, wrote, "I don't know who settled the score with the Iranian scientist, but I certainly am not shedding a tear."
Public reaction in Iran to the news varied widely. Messages to RFE/RL's Radio Farda ranged from denouncing the United States and Israel for the bombing to speculation that it was an inside job by the Iranian regime to mobilize support against the West.
Alireza Nourizadeh, a London-based commentator and director of the Center for Arab and Iranian studies, called the attack part of a "war" against Iran by countries determined to prevent Tehran from obtaining nuclear weapons.
He said the incident showed that "the war against Iran has begun. This war doesn't involve just one side. We can't tell if it was Israel, the United States, or anyone else. I think this recent event involved a host of countries."
But another analyst, Geneva-based Nima Rashedan, doubted that foreign countries were behind the bombing.
"I don't know how much you can trust reports" from Tehran blaming the West for the incident, Rashedan said, "when the Iranian government itself is not hiding the fact that it officially supports terrorist groups in various countries around the world."
This latest assassination is the fourth since the start of 2010 targeting Iranian physicists.
In July 2011, an Iranian university student was shot dead by a motorcyclist in Tehran outside his daughter's kindergarten, in what appeared to be a case of the assassin mistaking him for a physicist with an identical name.
In November 2010, a pair of back-to-back bombings killed one nuclear scientist and wounded another.
The wounded scientist, Fereidoun Abbasi, was later appointed as head of Iran's Atomic Energy Organization.
And in January 2010, a bomb explosion killed a senior physics professor at Tehran University.
International Tensions
The assassinations accompany rising international tensions as Iran continues to enrich uranium despite UN calls for it to suspend the activity, which can be used to produce nuclear fuel or, at high levels of enrichment, fissile material for bombs.
The UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), said on January 9 that Iran had started the production of uranium enriched up to 20 percent at its Fordow facility near the city of Qom, a site that had been kept secret until recently.
A 20 percent-enrichment level is far beyond the 4 percent level needed for nuclear fuel but still short of the 90 percent level needed for nuclear warheads.
The United States and its allies say Iran is using its nuclear program as a cover to develop atomic bombs.
Both the United States and the European Union recently expanded sanctions against Iran over its nuclear program.
Iran says its nuclear program is entirely for peaceful energy purposes.
In a speech broadcast on state television on January 9, Iranian Supreme Leader Ayatollah Ali Khamenei said further sanctions imposed by the West would not make Iran change its nuclear course.
