ISTANBUL — One of Iran's most prominent and well-guarded nuclear scientists was killed Friday in a daytime ambush on a rural road outside Tehran, an attack Iran's foreign minister blamed on Israel and that sharply raised regional tensions in the closing weeks of the Trump administration.

The scientist, Mohsen Fakhrizadeh, was seen as a driving force behind Tehran's disbanded effort to build a nuclear weapon nearly two decades ago. His role in Iran's current programs — reactors and uranium enrichment — was less direct and analysts said the killing would likely have a limited impact on Tehran's nuclear capabilities.

It also underscored one of the many challenges ahead for the Biden administration as it looks to reset U.S. policies toward Iran after President Trump's "maximum pressure" campaign.

Iran's foreign minister, Mohammad Javad Zarif, described the attack as the work of "state terror" and implicated Israel as having a possible role. Officials in Israel had no comment.

The attack — which Iranian news agencies said involved a car bomb and gunmen — recalled the shadowy killings of Iranian nuclear scientists a decade ago and exposed holes in Iran's security and intelligence agencies.

Just this year, it was the third high-profile attack to shake Tehran's leadership.

In January, a U.S. drone strike in Baghdad killed Maj. Gen. Qasem Soleimani, Iran’s most powerful military commander and head of its special-operations forces abroad. And in August, Israeli agents acting on behalf of American officials assassinated a senior al-Qaeda official in Tehran, according to a U.S. official.

Fakhrizadeh was once at the pinnacle of Iran’s nuclear program, including an effort to develop nuclear arms that U.S. intelligence says was scrapped in 2003. But his latest role was less directly involved in Iran’s nuclear sites, which include extensive centrifuge labs to enrich uranium.

While Fakhrizadeh had been a key figure in Iran’s bomb program, “that work is all in the past, and there is no reason to expect that if Fakhrizadeh is gone it would have any effect on Iran’s current nuclear program,” said Paul Pillar, a 28-year veteran of the CIA and a senior fellow at Georgetown University’s Center for Security Studies.

Analysts said the timing of the attack appeared linked to the impending change of U.S. administrations.

Trump — who withdrew the United States from a nuclear pact that Iran struck with world powers five years ago — has ramped up sanctions and other pressures on Tehran since walking away from the deal aimed at reining in Tehran’s nuclear program. Biden has pledged to work more closely with allies on Iran policies and work to rejoin the nuclear agreement.

Iran has recently increased its stockpile of enriched uranium since the Trump administration pulled out of the 2015 nuclear deal. Iran has insisted that the enriched uranium is intended only to power its nuclear energy plants and a research reactor. Iran’s foes counter that it puts the nation closer to producing warhead-grade material.

The Biden team also did not have an immediate comment. U.S. officials had no immediate comment, but Trump retweeted veteran Israeli journalist Yossi Melman, who described the attack as a “major psychological and professional blow for Iran.”

“The operation reflects thinking of those in the Netanyahu government — and/or the Trump administration — who see these next few weeks as their last chance to make relations with Iran as bad as possible, in an effort to spoil the Biden administration’s efforts to return to diplomacy with Tehran,” said Pillar, referring to Israeli Prime Minister Benjamin Netanyahu.

A Middle Eastern intelligence official said Israel was behind the attack. “There was an opportunity and it was taken,” the official said.

Former CIA director John O. Brennan, a strong Trump critic, tweeted that the attack was “a criminal act & highly reckless.”

“It risks lethal retaliation & a new round of regional conflict,” he wrote. “Iranian leaders would be wise to wait for the return of responsible American leadership on the global stage & to resist the urge to respond against perceived culprits.”

Accounts of Fakhrizadeh’s killing indicated his movements were being tracked and the attack was coordinated.

The semiofficial Tasnim news agency said the attack began with a car bomb that detonated in the path of Fakhrizadeh’s vehicle. Then “terrorists started shooting,” it reported.

But Brig. Gen. Amir Hatami, Iran’s defense minister, described a different chain of events in an interview with Iranian state television, saying the attack started with gunmen opening fire on Fakhrizadeh’s car. A pickup truck about 50 feet away exploded a short time later, he said. The gunfire continued, wounding the scientist and two of his bodyguards.

Fakhrizadeh later died at a hospital, the minister said.

A gray sedan, its windshield riddled by bullet holes, was shown in photos taken by Iranian news agencies at the scene of the attack.

The killing spurred calls from Iranian officials for accountability, or revenge.

“Terrorists murdered an eminent Iranian scientist today,” Zarif, Iran’s foreign minister, wrote on Twitter.

Iran has accused Israel and the United States of carrying out similar deadly attacks on nuclear experts in the past.

“This cowardice — with serious indications of Israeli role — shows desperate warmongering of perpetrators,” Zarif tweeted about the death of Fakhrizadeh. He said Iran calls on the international community, especially the European Union, “to end their shameful double standards & condemn this act of state terror.”

Hossein Dehghan, a former member of Iran’s paramilitary Revolutionary Guard and adviser to Iran’s supreme leader, wrote on Twitter: “We will descend like lightning on the killers.”

Fakhrizadeh “was one of the key individuals behind Iran’s nuclear program in the post-revolution era” and was deeply involved in shaping “the weapons phase of the program,” said Ariane Tabatabai, a Middle East fellow at the German Marshall Fund.

This is very significant politically and symbolically. It again exposes the deep flaws in Iran’s internal security. This is one of many incidents involving Iran’s nuclear program this year and one of several targeted killings on Iranian soil or affecting high-level Iranians,” she said.

Fakhrizadeh was widely regarded as the brains behind Iran’s nuclear program, including Tehran’s clandestine efforts to develop a nuclear bomb in the early 2000s. The physics professor, believed to be about 60 years old, has been identified by intelligence officials as the head of the Amad Plan, the secret nuclear weapons research program that sought to develop as many as six nuclear bombs before Iranian leaders ordered a halt to the program in 2003.

After the weapons program ended, he continued to supervise successor organizations that continued to employ many, if not most, of the Amad project’s scientists in conducting nuclear-related research, U.S. and Israeli analysts claim.

The current program is “now more focused on maintaining and developing nuclear weaponization capabilities rather than building the weapons themselves, said David Albright, president of the Institute for Science and International Security, a Washington nonprofit that tracks nuclear-weapons proliferation.

He called the attack on Fakhrizadeh a “shocking and disturbing development.”

Formerly a reclusive figure rarely seen in public, Fakhrizadeh has more recently allowed himself to appear on official Iranian websites, including during events held by Iran’s supreme leader, Ayatollah Ali Khamenei. Albright said the increased visibility “may have made him more vulnerable, making his movements easier to track.”

Targeted attacks between 2010 and 2012 killed at least four researchers and others with links to Iran’s nuclear program. Tehran accused Israel and the United States of masterminding the attacks as part of a covert war. U.S. officials have denied any role, and Israel has not commented.

In 2011, Darioush Rezaeinejad, an electrical engineer doctoral student whose work involved nuclear applications, was gunned down outside his Tehran apartment.

In 2012, motorcycle riders attached a magnetic bomb that tore apart a car carrying Mostafa Ahmadi Roshan, a nuclear scientist working at Iran’s main uranium-enrichment facility in Natanz. Roshan, 32, had planned to attend a memorial for another nuclear researcher, Tehran University professor Masoud Ali Mohammadi, who was killed in a similar pinpoint blast in 2010.

An Iranian man convicted in the Mohammadi attack, who Iran claimed was trained by Israel’s Mossad spy agency, was hanged in 2012.

Berger reported from Beirut and Warrick from Washington. Souad Mekhennet in Washington contributed to this report.