Fakhrizadeh, killed in an attack outside Tehran, was widely seen by Western intelligence as the mastermind of clandestine Iranian efforts to develop nuclear weapons. Here's more on the man whose killing is being blamed on Israel.
Prominent Iranian scientist Mohsen Fakhrizadeh, killed in an attack outside Tehran, has been widely seen outside the country as a leading figure in the Islamic Republic's nuclear weapons programme.
Iran denies his involvement.
Fakhrizadeh, once described by Israeli PM Benjamin Netanyahu as the father of Iran's nuclear weapons programme, was travelling in a car near Absard city in Tehran province's eastern Damavand county, when he was targeted.
Following his assassination on Friday, Iranian military and civilian officials immediately blamed Israel for the attack and warned that Fakhrizadeh's killing will be avenged.
What is known about Fakhrizadeh?
Western officials and experts believe Fakhrizadeh played a pivotal role in suspected Iranian work in the past to develop the means to assemble a nuclear warhead behind the facade of a declared civilian uranium enrichment programme.
Iran denies ever having sought to develop a nuclear weapon.
A landmark report by the UN nuclear watchdog in 2011 identified Fakhrizadeh as a central figure in suspected Iranian work to develop technology and skills needed for atomic bombs and suggested he may still have a role in such activity.
Believed to be a senior officer in the elite Revolutionary Guards, Fakhrizadeh was the only Iranian the report identified.
"If Iran ever chose to weaponise (enrichment), Fakhrizadeh would be known as the father of the Iranian bomb," a Western diplomat told Reuters news agency in 2014.
The UN report said he oversaw activities "in support of a possible military dimension to (Iran’s) nuclear programme" within the so-called AMAD Plan.
A 2011 IAEA report described him as the AMAD Plan's "Executive Officer", a central figure in suspected Iranian work to develop technology and skills needed for atomic bombs, and suggested he may still have a role in such activity.
What does Iran say?
The UN International Atomic Energy Agency (IAEA) has long wanted to meet Fakhrizadeh as part of a protracted investigation into whether Iran carried out illicit nuclear weapons research.
Showing no sign it would heed the request, Iran acknowledged Fakhrizadeh's existence several years ago but said he was an army officer not involved in the nuclear programme, according to a diplomatic source with knowledge of the matter.
He was also named in a 2007 UN resolution on Iran as a person involved in nuclear or ballistic missile activities.
What is known about his background?
An exiled Iranian opposition group, the National Council of Resistance of Iran (NCRI), in May 2011 issued a report with what it said was a photograph of Fakhrizadeh, with dark hair and beard stubble.
It was not possible to independently verify the picture.
The NCRI said in the report that Fakhrizadeh was born in 1958 in the city of Qom, was a deputy defence minister and a Revolutionary Guards brigadier-general, and holds a nuclear engineering doctorate and taught at Iran's University of Imam Hussein.
A high-ranking Iranian source described Fakhrizadeh to Reuters in 2014 as "an asset and an expert" dedicated to Iran's technological progress and enjoying the full support of Iranian Supreme Leader Ayatollah Ali Khamenei.
The source added that Fakhrizadeh had three passports and travelled a lot, including in Asia, to obtain "the latest information" from abroad, but would not elaborate.
Western security sources say Iran was long adept in obtaining nuclear materials and know-how from the international black market.
Is Israel behind his assassination?
Israel has described the AMAD Plan as Iran's covert nuclear weapons programme, and says it seized a large chunk of an Iranian nuclear "archive" detailing its work.
In an April 2018 televised presentation about the archive, Israeli Prime Minister Benjamin Netanyahu named Fakhrizadeh as a leading figure in what he described as secret nuclear weapons work conducted under the guise of a civilian programme.
"Remember that name, Fakhrizadeh," he said in a 2018 speech revealing details from the archive, naming Fakhrizadeh as Amad's chief.
In 2018 Israeli broadcaster Kan carried an interview with former Prime Minister Ehud Olmert in which he hinted Fakhrizadeh could be a target.
"I know Fakhrizadeh well. He doesn't know how well I know him. If I met him in the streets most likely I would recognise him," Olmert said.
"He does not have immunity, he did not have immunity, and I don’t think he will have immunity."
Immediately after his killing, Iran's Foreign Affairs Minister Javad Zarif tweeted that there were "serious indications of Israeli role" in Fakhrizadeh's assassination.
Israel has not commented on Iranian allegations.