Iranian security forces inspect the site where a magnetic bomb attached to a car by a motorcyclist exploded outside a university in Tehran on Wednesday, killing a scientist and injuring two other people, according to Iranian news agencies. (Sajad Safari/AFP/Getty Images)

A scientist linked to Iran’s nuclear program was killed in his car by a bomb-wielding assailant on Wednesday, a bold rush-hour attack that experts say points to a further escalation in a covert campaign targeting the country’s atomic officials and institutions.

The precision hit in a northern Tehran neighborhood killed the 32-year-old chemical engineer employed at Iran’s main uranium-enrichment facility and brought to four the number of Iranian scientists killed by bombs in the past two years. No one asserted responsibility for the bombing, which prompted a swirl of accusations and denials as well as renewed concerns about worsening tensions between Iran and the West.

Iranian officials immediately accused the United States and Israel of orchestrating the attack on scientist Mostafa Ahmadi-Roshan, who was killed along with his bodyguard when an assailant on a motorcycle slapped a magnetic bomb on his car as he commuted to work, according to Iranian news reports. Vice President Mohammad Reza Rahimiblamed the attack on “Zionists” and “those who claim they are against terrorism,” the semiofficial Fars News Agency reported.

The Obama administration denied involvement in the attack and distanced itself from the kind of lethal tactics used to kill the scientist.

“I want to categorically deny any United States involvement in any kind of act of violence inside Iran,” Secretary of State Hillary Rodham Clinton told reporters shortly after the bombing was reported.

Israeli officials declined to address Iranian accusations linking Israeli intelligence operatives to the hit. “It is not our policy to comment on this sort of speculation when it periodically arises,” an Israeli official said, speaking on the condition of anonymity under government ground rules.

But the series of attacks against scientists — all of them employed in fields or institutions relevant to Iran’s nuclear program — underscored the perception of a sophisticated covert campaign to disrupt Iran’s nuclear efforts and intimidate key officials and scientists, according to security analysts and Iran experts. The killing bore strong resemblance to two 2010 attacks on nuclear scientists and came on the same day as a ceremony for the second anniversary of the killing of another professor, Massoud Ali Mohammadi, in an explosion.

The scientists’ deaths are part of a pattern of attacks and apparent sabotage. In recent years, Iran has experienced an increase in mysterious explosions at military and industrial sites and gas transportation lines. A computer virus called Stuxnet also has damaged the nation’s nuclear program.

“The idea clearly is to try to disrupt operations that could lead to a nuclear weapon, and to make their scientists feel less secure and less capable of doing their work,” said David Albright, a former U.N. weapons inspector and president of the Institute for Science and International Security.

Some current and former government officials worried that the tactics could backfire, bolstering Iran’s resolve to defy the West.

“It certainly increases their paranoia,” said Robert Baer, a former CIA case officer in the Middle East. “In any case, they’re not going to stop their program just because a scientist is dead.”

Among Western officials and experts, speculation about the possible culprit included Israel’s Mossad intelligence agency, Iranian opposition groups and Sunni Arab governments seeking to thwart the ambitions of Iran’s Shiite rulers.

In Israel, among those commenting on the attack was Brig. Gen. Yoav Mordechai, chief military spokesman, who made an unusual posting on his official Facebook page hours after the killing: “Don’t know who settled the score with the Iranian scientist, but for sure I am not shedding a tear.”

The posting sparked a debate on his page, with some readers saying he should be more discreet.

Iran filed a formal complaint about the attack late Wednesday with the United Nations, saying its scientists were victims of “cruel, inhumane and criminal acts of terrorism.”

“There is firm evidence that certain foreign quarters are behind such assassinations,” Iran’s ambassador to the United Nations, Mohammad Khazaee, wrote to U.N. Secretary General Ban Ki-moon. He added that Iran would not be pressured into giving up its “inalienable right to use nuclear energy for peaceful purposes.”

In the Iranian capital, members of parliament reacted to news of the bombing by shouting “Death to America!” and pumping their fists in the air in a show of defiance. Parliament’s national security and foreign policy committee called an emergency session to discuss a response to the “terrorist act,” the official Islamic Republic News Agency reported.

One analyst who supports Iran’s leaders said it was time for Iran to strike back. “From now on, the reciprocation is internationally acceptable and legitimate,” said Mehdi Mohammadi, an international affairs analyst who regularly appears on state television. “Iran is for sure able to enter a new phase of confrontation,” he told the Asriran Web site.

The attack occurred against the backdrop of a worsening international crisis over Iran’s nuclear program. The United States and its allies are preparing to implement harsh new economic sanctions against the Islamic republic, prompting a series of threats and provocations by Iranian officials. In the past two weeks, Iran threatened a U.S. aircraft carrier and vowed to shut down shipping in the strategic Strait of Hormuz. The nation also has sentenced to death a former U.S. Marine accused of spying for the CIA.

The Obama administration has been pressing allies to curb imports of Iranian petroleum, the country’s economic lifeblood.

The scientist targeted in Wednesday’s bombing was described by Iranian media as the deputy director of Iran’s largest uranium-enrichment facility, near the town of Natanz.

Fars said an assailant on a motorcycle attached the bomb to Ahmadi-Roshan’s car, in the attack caught on traffic-control cameras. Another news Web site, Alef, said witnesses heard gunshots before the explosion.

The site of Wednesday’s blast was cordoned off by security forces, who stopped and searched bystanders. The damaged car, a locally made Peugeot 405, was quickly removed.

Warrick reported from Washington. Special correspondents Ramtin Rastin in Tehran and Samuel Sockol in Jerusalem contributed to this report.