The United States called "absurd" charges by Iran that it had anything to do with the Tuesday motorcycle bomb blast that killed a prominent nuclear physics professor who publicly backed opposition leader Mir Hossein Mousavi in the June presidential election.
State media identified Masoud Ali Mohammadi as the victim of a bomb-rigged motorcycle that blew up outside his home.
Mohammadi, 50, was a professor at Tehran University, which has been at the center of recent protests by student opposition supporters. Before the election, pro-reform Web sites published Mohammadi's name among a list of 240 university teachers who supported Mousavi.
The government blamed the attack Tuesday on an armed Iranian opposition group under the direction of Israel and the U.S.
Hard-line government supporters called at recent street rallies for the execution of the opposition leaders.
Mohammadi had just left his house on his way to work when the remote-controlled explosion went off, state TV said. The blast shattered the windows of his home in northern Tehran's Qeytariyeh neighborhood and left the pavement outside smeared with blood and strewn with debris.
State media blamed the killing on the West, which is locked in a tense confrontation with Iran over its nuclear program. One news Web site associated with a prominent member of the country's clerical leadership singled out the United States and Israel, saying the assassination was probably the work of an armed Iranian opposition group under the direction of Israeli agents.
In Washington, State Department spokesman Mark Toner denied the charge.
"Any charges of U.S. involvement are absurd," he said. Israel's Foreign Ministry had no comment.
The semiofficial ISNA news agency quoted Tehran prosecutor Abbas Jafari Dolatabadi as confirming the killing and saying no one has been arrested.
"Most probably, the intelligence services and elements of (Israel's) Mossad and the CIA had a hand in his assassination," the prosecutor said, according to the Web site of state TV.
A spokesman for the atomic agency, Ali Shirzadian, told The Associated Press that Ali Mohammadi had no link with the agency responsible for Iran's contentious nuclear program. Iran is under pressure from the United States and its European allies, which suspect Tehran is seeking to develop nuclear weapons. Iran denies that.
"He was not involved in the country's nuclear program," Shirzadian said of the victim.
Iran denies having any intention to produce weapons and insists its nuclear work only has peaceful aims, such as energy production.
Mohammadi was the author of several articles on quantum and theoretical physics in scientific journals. He also was a member of some academic associations focusing on experimental science, but he did not appear to have any high-profile role in promoting Iran's nuclear program.
He received his doctorate in 1992 from the Sharif University of Technology in Tehran.
Iran's suspicions for the assassination fell on exiled opposition group the People's Mujahedeen Organization of Iran and Israeli agents, said the news Web site Tabnak. The Tabnak site is closely associated with Mohsen Rezaei, who serves on an advisory body to the country's supreme leader, Ayatollah Ali Khamenei.
The Web site of Iran's state television declared the bombing a "terrorist act by counter revolutionaries and elements of arrogance," a reference to the United States. Security forces are investigating, the report said.
In 2007, state TV reported that another nuclear scientist, Ardeshir Hosseinpour, died from gas poisoning. A one-week delay in the reporting of his death prompted speculation about the causes, including that Israel's Mossad spy agency was to blame.
Ali Mohammadi wrote several articles on quantum and theoretical physics in scientific journals.
He was not known as an outspoken or visible supporter of Iran's opposition movement in the postelection turmoil, though his name did appear on the list of professors who backed Mousavi before the vote. That list was published on several pro-reform Web sites in the weeks leading up to the election.
Mousavi and his supporters claim he was true winner of the June election but fraud robbed him of his victory. His supporters staged massive street protests in the weeks after the election, which met with a harsh government crackdown.
The semiofficial Mehr news agency quoted a Tehran University official as saying Ali Mohammadi was not involved in political activity.
"The prominent professor was not a political figure and had no activity in the field of politics," Mehr quoted Ali Moqari, head of the university's science department, as saying.
Ali Mohammadi was a member of some academic associations focusing on experimental science. In 1992, he received the first doctorate in nuclear physics to be awarded in Iran, from Tehran's Sharif University of Technology.
Iran's Foreign Ministry accused Israel and the U.S. of involvement in the assassination, according to the Web site of state TV.
"In initial investigations, there are some indications of vices of the Zionist regime, the U.S. and their mercenaries in Iran in the terrorist incident," ministry spokesman Ramin Mehmanparast was quoted as saying in the report.
He said the killing of nuclear scientists cannot thwart the country's scientific and technological progress.
Iran also directed suspicion at the exiled opposition group the People's Mujahedeen Organization of Iran. Tabnak, a conservative Iranian Web site close to the ruling establishment, said the group carried out the attack under direction of Israeli agents.
The People's Mujahedeen, however, denied any involvement and said the claim was a government ploy to smear the group.
The Basij militia, which along with its allied Revolutionary Guard played a key role in the postelection crackdown on the opposition, condemned the killing and called the professor a martyr.
The Basij said Ali Mohammadi also taught at the Imam Hossein and Malek Ashtar universities, both linked to the Revolutionary Guard.
The United States and its allies in Europe have been pushing Iran to halt its uranium enrichment program, a technology that can be used to make fuel for power plants but which also offers a possible pathway to weapons development.
Israel has threatened to take military action if diplomatic efforts fail.
U.S. Secretary of State Hillary Rodham Clinton said Monday that the Obama administration has concluded that the best way to pressure Iran to come clean on its nuclear ambitions is to impose sanctions aimed at the country's ruling elite.
Iran is already under three sets of U.N. sanctions for refusing to freeze its enrichment work.
The Associated Press contributed to this report.