Iran's top nuclear scientist Mohsen Fakhrizadeh was assassinated near the capital Tehran on Friday.
The Iranian state-run media IRNA reported that he had been assassinated in a terror attack in which the sound of an explosion was followed by gun shots, according to witnesses.
Fakhrizadeh is known to have led the development of Iran's secret nuclear weapons that started in 1999 and halted in 2003.
The UN speculated in their 2011 report that he could still be trying to acquire technology for nuclear weapon development.
Iran's foreign minister Mohammad Javad Zarif tweeted that Israel could be behind the attack and that such a cowardly act is a war provocation.
He also urged global society, especially the EU, to drop their embarrassing double standards, and denounce such terror.
The U.S. and Israel's defense ministry did not comment on his death.