Prominent Iranian scientist Mohsen Fakhrizadeh, killed in an attack outside Tehran on Friday, was widely seen outside the country as a leading figure in the Islamic Republic's nuclear program. Iran denies his involvement.
What do we know about him?
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 program.
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.
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.
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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 program, 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 do we know 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 holy Shi'ite Muslim city of Qom, was a deputy defense minister and a Revolutionary Guards brigadier-general, and holds a nuclear engineering doctorate and taught at Iran's University of Imam Hussein.
Fakhrizadeh was the latest in a string of Iranian nuclear scientists to be attacked since 2012. Here are some details of other attacks on Iranian scientists in recent years:
Nuclear scientist Massoud Ali-Mohammadi was killed by a remote-controlled bomb in Tehran on January 12, 2010. Some opposition websites said he had backed moderate candidate Mirhossein Mousavi in the disputed 2009 election that secured a second presidential term for Mahmoud Ahmadinejad.
Iranian officials described the physics professor as a nuclear scientist but a spokesman said he did not work for the Atomic Energy Organisation. He lectured at Tehran University.
Western sources said the professor worked closely with Fakhrizadeh and Fereydoun Abbassi-Davani, who were both subject to UN sanctions because of their work on suspected nuclear weapons development.
A list of Ali-Mohammadi's publications on Tehran University's website suggested his specialization was theoretical particle physics, not nuclear energy, a Western physics professor said.
Shahriyari was killed and his wife was wounded in a car bomb blast in Tehran on November 29, 2010, in what Iranian officials called an Israeli or U.S.-sponsored attack on its atomic program.
Iran's atomic energy agency chief Ali Akbar Salehi said Shahriyari had a role in one of its biggest nuclear projects, but did not elaborate, the official news agency IRNA reported. He was a lecturer at Shahid Beheshti University.
Abbasi-Davani and his wife were hurt in a car bomb blast on the same day as Shahriyari was killed.
Abbasi-Davani, who was head of physics at Imam Hossein University, had been personally subject to U.N. sanctions because of what Western officials said was his involvement in suspected nuclear weapons research.
The intelligence minister at the time, Heydar Moslehi, later said: "This terrorist act was carried out by intelligence services such as the CIA, Mossad and the MI6. A group that wanted to carry out a terrorist act but did not succeed, was also arrested. They confessed that they were trained by these intelligence services."
Abbasi-Davani was appointed vice-president and head of the Atomic Energy Organization in February 2011, Fars news agency reported, but was removed in August 2013, the state news agency IRNA reported.
Rezai, 35, was shot dead by gunmen in eastern Tehran on July 23, 2011. The university lecturer had a PhD in physics. Deputy Interior Minister Safarali Baratlou said he was not linked to Iran's nuclear program after early reports in some media said he was.
Ahmadi-Roshan, a 32-year-old chemical engineering graduate, was killed by a bomb placed on his car by a motorcyclist in Tehran in January 2012. Another passenger died in hospital and a pedestrian was also injured. The attack was similar to that in November 2010.
Iran said the victim was a nuclear scientist who supervised a department at Natanz Uranium Enrichment Facility. Iran blamed Israel and the United States for the attack.