Iran is blaming the murder of Mostafa Ahmadi Roshan on Israel and the United States. Roshan was a chemistry expert and director of the Natanz uranium enrichment facility in central Iran.
Two assailants on a motorcycle reportedly attached a magnetic bomb to Roshan's car, killing him and his driver on Wednesday. The attack bore a strong resemblance to earlier killings of scientists working on the Iranian nuclear program.
The attack is leading to even higher tensions between the U.S. and Iran.
The U.S. and its allies have been pressuring Iran to halt uranium enrichment, a key element of the nuclear program that the West suspects is aimed at producing atomic weapons. Uranium enriched to low levels can be used as nuclear fuel but at higher levels, it can be used as material for a nuclear warhead.
Iran continues to deny it is trying to make nuclear weapons, saying its program is for peaceful purposes only and is geared toward generating electricity and producing medical radioisotopes to treat cancer patients.
The Associated Press contributed to this report.