For years, Mohsen Fakhrizadeh balanced his high status in Iran’s nuclear weapon program with a low public profile. The Iranian state media rarely acknowledged him, and he was sometimes described as simply a university professor.

It was only long after Iran’s nuclear weapons program was formally abandoned that Western organizations publicly identified him as a key figure in the effort. “Remember that name, Fakhrizadeh,” Israeli Prime Minister Benjamin Netanyahu said during a presentation on Iran’s covert program in 2018.

But in recent years, Fakhrizadeh’s reticence toward publicity appeared to have slipped. He appeared on official Iranian websites at events with Iran’s supreme leader, Ayatollah Ali Khamenei. Ultimately, that may have been his downfall.

As reports of his killing Friday during an ambush outside Tehran are detailed, Fakhrizadeh’s name is becoming even more well-known. But even in death, there remains an air of mystery surrounding the Iranian nuclear scientist and what effect his killing will have on Iran.

“Fakhrizadeh likely knew more about Iran’s nuclear program than any living human. Losing his leadership, knowledge and institutional memory is undoubtedly a blow to the Islamic Republic,” said Karim Sadjadpour, a senior fellow at the Carnegie Endowment for International Peace.

Some analysts compared the importance of Fakhrizadeh’s death within Iran to that of the more well-known Qasem Soleimani, the notorious — and for many iconic — leader of Iran’s clandestine operations abroad, who was assassinated in a U.S. airstrike while visiting Baghdad in January.

“They were parallel in terms of seniority and prestige within Iran, though they were doing two totally different things,” said Simon Henderson, an expert on the Middle East at the Washington Institute for Near East Policy.

But among experts on nuclear proliferation, there were doubts that the death of a man best known for decades-old work would have a significant impact on contemporary weapons programs.

“Plenty of people in the program can design a warhead now if needed,” said Vipin Narang, an associate professor of political science at the Massachusetts Institute of Technology. “So Iran may now have more motivation to weaponize, without attenuating its ability to do so.”

“These sorts of people are significant but ultimately not irreplaceable,” Henderson said, noting that Soleimani’s killing had not ended Iranian operations abroad.

Like Soleimani, Fakhrizadeh was a young man during the Iranian Islamic Revolution that overthrew Shah Mohammad Reza Pahlavi. He is thought to have been about 60 when he died, which would have made him 18 or 19 during the events of 1979.

After the revolution, he joined the Islamic Revolutionary Guard Corps, a new group designed to protect the new republic and enforce its strict ideological aims.

Eventually, he would become a key figure in its drive for a nuclear weapon. As a former leader of Iran’s military-linked Physics Research Center, he was involved in drawing up plans and acquiring parts for Iran’s first uranium enrichment plant, according to U.N. officials.

Under pressure from Western nations, Iran’s secret nuclear weapons program was formally halted in 2003. While Fakhrizadeh was never interviewed by investigators from the International Atomic Energy Agency, in the years since, he has been recognized as the leader of the program, known as the Amad Plan.

He was among eight Iranians placed under international travel and financial restrictions under the terms of a U.N. resolution adopted in 2007 because of his alleged ties to “nuclear or ballistic missile” research, U.N. records show.

His importance to Iran’s weapons program is underscored in thousands of Iranian documents that were stolen by Israeli operatives and smuggled out of the country in 2018. The documents and other records portray Fakhrizadeh as the project’s leader since 1998.

With his presentation of the documents in 2018, Netanyahu showed the first known photograph of Fakhrizadeh and dubbed him a “shadow man” in Iran’s nuclear weapons effort.

After the weapons program was halted, Fakhrizadeh continued to supervise successor organizations that continued to employ many, if not most, of the Amada Plan’s scientists in conducting nuclear-related research, U.S. and Israeli analysts claim.

A June 2020 report released by the U.S. State Department stated that former workers in the nuclear weapons program had been kept employed on projects with “weaponization-relevant dual-use technical activities” under the leadership of Fakhrizadeh.

This suggests that “Iran preserved this information at least in part potentially to aid in any future nuclear weapons development work in the event that a decision were made to resume such work,” the State Department wrote.

Fakhrizadeh’s death drew widespread coverage in Iranian media, where he was described as head of the Research and Innovation Organization in the Iranian Defense Ministry. Some accounts said he had been involved in the Iranian military’s response to the coronavirus pandemic.

“Iran has a history of elevating the status of Iranian officials killed in service of the country” to martyrs, said Eric Brewer, an expert on nuclear weapons at the Center for Strategic and International Studies. Tehran is “going to have to walk a fine line on that with Fakhrizadeh, though, because it denies there was ever a weapons program,” Brewer added.

But even if his work was covert, his legend is now growing. Holly Dagres, a fellow at the Atlantic Council, said he was thought of as an “Iranian Robert Oppenheimer,” a reference to the scientist who developed the world’s first nuclear weapon for the United States.

Dagres noted, however, that Iran had never actually created a nuclear weapon. She doubted Fakhrizadeh’s death would permanently damage Iran’s nuclear program. “Iran’s nuclear know-how isn’t dependent on one man,” she said.

What may be more damaging is the idea that Fakhrizadeh, probably one of Iran’s most heavily protected nuclear scientists, could be killed on Iranian soil. No country had claimed responsibility for the killing, though Israel has been accused of killing Iranian nuclear scientists in the past.

“Soleimani’s death didn’t end Iran’s regional activities and ambitions, but it was demoralizing for the Revolutionary Guards and Iran’s regional proxies, and his successor has been incapable of filling his large shoes,” Sadjadpour said. “Fakhrizadeh’s death could have a similar impact.”

Miriam Berger and Joby Warrick contributed to this report.