An Iranian university professor working at a key nuclear facility was killed today by a car bomb, a semi-official news agency reported.
The explosion killed Mostafa Ahmadi Roshan, a chemistry expert and a director of the Natanz uranium enrichment facility in central Iran, the Fars agency said.
A senior Iranian official immediately blamed Israel.
"The responsibility of this explosion falls on the Zionist regime," the governor of Tehran province, Safar Ali Bratloo, told Iran's Arabic-language Al-Alam broadcaster.
"The method of this terrorist action is similar to previous actions that targeted Iran's muclear scientists," he said.
Two assailants on a motorcycle attached magnetic bombs to Mr Roshan's car, killing him and wounding two others in the Iranian capital.
The attack in Tehran strongly resembles earlier killings of scientists working on the country's controversial nuclear programme.
The killing of Mr Roshan was similar to previous assassinations of Iranian nuclear scientists that Tehran has blamed on Israel and the United States. Both countries have denied the accusations.
Mr Roshan, 32, was inside the Iranian-assembled Peugeot 405 car with two others when the bomb exploded near Gol Nabi Street in north Tehran, Fars said.
Fars described the explosion as a "terrorist attack" targeting Mr Roshan, a graduate of the prestigious Sharif University of Technology in Tehran.
A similar bomb explosion on January 12, 2010 killed Tehran University professor Masoud Ali Mohammadi, a senior physics professor. He was killed 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 one nuclear scientist and wounded another.
The scientist killed in those attacks, Majid Shahriari, was a member of the nuclear engineering faculty at Shahid Beheshti University in Tehran and co-operated with the Atomic Energy Organisation of Iran. The wounded scientist, Fereidoun Abbasi, was almost immediately appointed head of Iran's atomic agency.
In July last year, 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.
Mr Rezaeinejad allegedly participated in developing high-voltage switches, a key component in setting off the explosions needed to trigger a nuclear warhead.
The United States and other countries say Iran is trying to develop nuclear weapons technology. Iran denies the allegations, saying that its programme is intended for peaceful purposes.