Two Iranian academics, who would speak only anonymously for fear of official reprisals, said in telephone interviews that he had specialized in particle and theoretical physics. The Web site of Tehran University lists him as a professor of elementary particle physics.

A spokesman for Iran’s atomic agency, Ali Shirzadian, told The Associated Press that Mr. Ali Mohammadi had no link with the agency responsible for Iran’s nuclear program.

While Press TV called the professor a “staunch supporter of the Islamic Revolution” of 1979, that claim seemed dubious.

Ali Moghari, the director of the science department of Tehran University, described Mr. Ali Mohammadi as an “apolitical professor,” the semiofficial Mehr news agency reported. “He was a well-known professor but was not politically active,” he was quoted as saying.

There were some indications that he might have been taking a more active role in the opposition that sprang up after the flawed presidential election last June. Mr. Ali Mohammadi was among 240 university professors who signed a letter before the election expressing support for the main opposition candidate, Mir Hussein Moussavi.

After a brutal crackdown, the authorities late last year broadened efforts to stifle dissent to encompass the educational system, hinting that dissident professors would be purged. A number of hard-line clerics have called for the university humanities curriculums to be further Islamized. But it was not immediately known whether Tuesday’s killing was related to that dispute.

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Analysts said the Iranian authorities seemed to have been quick to label Mr. Ali Mohammadi a loyalist, possibly as a precursor to renewed, harsh action against their opponents.

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“This is an old trick,” said a former senior official, who spoke on the condition of anonymity for fear of retribution. “They did it themselves but blame it on opposition groups so that they can easily begin issuing death sentences for protesters. I think this means there could be more violence against the opposition.”

The authorities have already announced plans to try five protesters on a charge of “fighting against God,” which carries a death sentence for those who are convicted.

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Seven leaders of the minority Bahai religious group were tried Tuesday in Tehran in closed proceedings, the Human Rights Activist News Agency, an opposition Web site, reported. The seven, who have been in jail for more than 20 months, were charged with committing religious offenses and trying to disrupt national unity. Recently, Iranian officials have accused Bahai leaders of fomenting the street demonstrations.

Last week, pro-government demonstrators shot at the armored car of Iran’s most outspoken opposition leader, Mehdi Karroubi, his Saham News Web site reported.

That attack appeared to reflect growing frustration that the crackdown in recent months had failed to stop the opposition from lashing out at the country’s leaders and staging intermittent protests that brought tens of thousands of demonstrators into the streets.

The Web site of Iran’s state television declared the bombing a “terrorist act by counterrevolutionaries and elements of arrogance,” a reference to the United States. Security forces are investigating, The Associated Press quoted the report as saying.

Last year, an Iranian nuclear scientist, Shahram Amiri, disappeared during a pilgrimage to Saudi Arabia; Iran accused the United States of helping to kidnap him.

The United States and its Western allies have been pressing Iran to halt its nuclear enrichment program, which Tehran insists is solely for civilian purposes to produce electricity. But the West fears that Iran is trying to build a nuclear weapon, which would threaten Israel and upset the regional power balance.

Speaking Monday at the start of a nine-day trip across the Pacific, Secretary of State Hillary Rodham Clinton said the United States and its allies were discussing financial sanctions that would appear to be aimed at Iran’s Revolutionary Guards and other political players if diplomacy fails to overcome the growing tensions with Iran.

“It is clear that there is a relatively small group of decision makers inside Iran,” she said. “They are in both political and commercial relationships, and if we can create a sanctions track that targets those who actually make the decisions, we think that is a smarter way to do sanctions.”

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But, she added, “all that is yet to be decided upon.”