Here's what you need to know.

What happened?

Mohsen Fakhrizadeh, believed to be the mastermind of Iran's controversial nuclear program, was in a car east of Tehran when he was shot dead on Friday afternoon. While there are conflicting reports on how the attack unfolded, most accounts agree that it was a sophisticated attack with gunfire and an explosion. But at least one account offered even more dramatic claims.

The semi-official Fars News Agency claimed on Sunday that Fakhrizadeh was traveling with his wife in a bulletproof car, alongside three security personnel vehicles, when he heard what sounded like bullets hitting a vehicle and got out of his car to find out what happened.

At that point, a remote-controlled machine gun opened fire from a Nissan stopped about 150 meters (164 yards) from Fakhrizadeh's car, Fars claimed. The scientist was hit at least three times and the Nissan exploded after the attack.

Weapons experts say the claim is technically possible. But experts agree it is unlikely that those behind the assassination would choose this method because of its high risk of failure. Iranian news agencies have also offered contradictory versions of events.

It is also possible that Iranian authorities are trying to exaggerate the sophistication of the attack to play down the glaring weakness of its security apparatus exposed by the assassination in broad daylight of one of its top officials. The killing has left Iran feeling exposed and vulnerable.

Who was he and why was he important?

It's difficult to overstate Fakhrizadeh's importance. Western intelligence agencies consider him to be the father of Iran's nuclear ​weapons program, the alleged existence of which has been at the heart of Tehran's standoff with the international community for nearly two decades. Iran has repeatedly maintained that its nuclear program has been used ​exclusively for peaceful purposes, but Western states accuse Tehran of seeking to develop a nuclear bomb. If that ever came to fruition, intelligence agencies say, it would have been Fakhrizadeh's brainchild.

In 2015, the Obama administration clinched a landmark deal to curb Iran's nuclear program in exchange for sanctions relief, but it was short-lived. In 2018, Trump pulled out of the pact and meted out some of the most crippling sanctions Tehran has ever been subjected to. A year later, Iran resumed enriching uranium, and took other steps to reactivate its nuclear program. This alarmed Western capitals but experts believe Iran is still years away from developing a nuclear weapon.

While Fakhrizadeh was a key player in Iran's nuclear program, he was an old hand, so his expertise is unlikely to have died with him.

Who did it?

Iran has blamed Israel for the attack and said the operation bore the hallmarks of Israel's foreign intelligence agency, the Mossad. While Iran has provided no evidence of Israeli involvement, Israel has neither denied nor claimed responsibility for Fakhrizadeh's death.

In 2018, Israeli Prime Minister Benjamin Netanyahu said Fakhrizadeh was the head of Project Amad, which he and others describe as a secret nuclear weapons endeavor.

"Remember that name, Fakhrizadeh," the prime minister told reporters at the time.

Shortly after the assassination, US President Donald Trump retweeted Israeli journalist Yossi Melman, who wrote that Fakhrizadeh "was head of Iran's secret military program and wanted for many years by Mossad."

What has Iran said, and what does it mean for the nuclear deal?

The Iranians have vowed retribution, but it's unclear​ how, or whether​, this will materialize. After ​Trump ordered the killing of top Iranian general Qasem
Soleimani in January, Tehran raised the specter of a cataclysmic retaliation. The Iranians responded quickly, but with restraint.

In January, Iran fired several rounds of ballistic missiles at US positions in nearby Iraq, but gave troops advance warning, enabling them to take cover in bunkers. But in addition to that, a military error that night led to the shooting down of a Ukrainian commercial plane that had taken off from Tehran's international airport, killing 176 people on board.

Iran's restraint after Soleimani's killing was seen as part and parcel of President Hassan Rouhani's policy of "strategic patience." For years, Iranians have been banking on a Democrat returning to the White House -- and they are counting on US President-elect Joe Biden to make good on his promise to restore the nuclear deal.

What happens to the nuclear deal hinges on Iran's response to this assassination. If Iran retaliates now and sets off a conflict, it would make it more difficult for all sides to get back to the negotiating table next year.

But Iran's patience is wearing thin. Its economy has suffered under unrelenting sanctions and its top leadership was dealt repeated blows in recent years.

Among lawmakers, calls for retaliation are growing louder, and Tehran's deterrent powers have been greatly diminished by expectations of restraint.
Whether or not Tehran retaliates, it has much to lose, and it will tread carefully.