Iran’s top nuclear scientist, who American and Israeli intelligence have long charged was behind secret programs to design an atomic warhead, was shot and killed on Friday as he was traveling in a vehicle in northern Iran, Iranian state media reported.
The scientist, Mohsen Fakhrizadeh, believed to be 59, has been considered the driving force behind Iran’s nuclear weapons program for two decades, and continued to work after the main part of the effort was quietly disbanded in the early 2000s, according to American intelligence assessment.
Mr. Fakhrizadeh was shot as his car was driving through the countryside town of Absard, in the Damavand region, according to official Iranian media and state television.
The state media accounts said that Mr. Fakhrizadeh had been gravely wounded in the attack, and that doctors tried to save him in the hospital but could not.
The assassinated scientist had long been a target of Israel’s Mossad.
A shadowy figure, Mr. Fakhrizadeh had long been the No. 1 target of the Mossad, Israel’s intelligence service, which is widely believed to be behind a series of assassinations more than eight years ago that included some of Mr. Fakhrizadeh’s deputies.
Iran never agreed to demands from the International Atomic Energy Agency, the United Nations’ nuclear monitoring agency, to let their inspectors question Mr. Fakhrizadeh, saying he was an academic who lectured at the Imam Hussein University in downtown Tehran.
Mr. Fakhrizadeh was an academic, but a series of classified reports, notably a lengthy 2007 assessment done by the C.I.A. for the George W. Bush administration, said the academic role was a cover story. In 2008, his name was added to a list of Iranian officials whose assets were ordered frozen by the United States.
That same year, his activities were disclosed in an unclassified briefing by the I.A.E.A.’s chief inspector. Later, it became clear that he ran what the Iranians called Projects 110 and 111 — an effort to tackle the most difficult problems bomb designers face as they try to make a warhead small enough to fit atop a missile and make it survive the rigors of re-entry into the atmosphere.
Iran has always denied it was seeking a nuclear weapon, insisting its production of nuclear material was purely for peaceful purposes.
The killing is bound to provoke sharp reaction in Iran.
Mr. Fakhrizadeh’s killing, whoever was responsible, could have broad implications for the incoming Biden administration. It is bound to set off a sharp reaction in Iran, as did the American attack on Jan. 3 that killed Qassim Suleimani, the Iranian major general who ran the elite Quds force of the Islamic Revolutionary Guards Corps.
The killing of Mr. Fakhrizadeh could complicate the effort by President-elect Joseph R. Biden Jr. to revive the 2015 Iranian nuclear deal, as he has pledged to do, if the Iranians agree to return to the limits detailed in the accord.
Israel has long opposed the deal, and if its agents were responsible for the killing of a man considered a national hero — in Iran, they will almost certainly be accused of being behind it — there could be political pressure in Iran to move forward with its current effort to gradually rebuild the stockpile of nuclear fuel that it gave up in 2015.
American officials would not comment on the assassination on Friday morning, saying they were seeking information.