The killing of Ayatollah Ali Khamenei leaves an enormous power vacuum in Iran, and no-one really knows who'll fill it.
Ayatollah Ali Khamenei was called the "supreme leader" of Iran for a reason.
Ayatollah translates to "sign of God". There are many ayatollahs in Iran, but the supreme leader is at the very top of the hierarchy, above the president.
"He dominated all aspects of life," says Ali Vaez, Iran director with the International Crisis Group.
Until his assassination in Israeli and US air strikes on the weekend, Khamenei had run Iran for 37 years. His death leaves an enormous power vacuum.
After all, the Iranian constitution gives the supreme leader "absolute authority" over the legislative, executive, and judicial branches of government. He determined the overall politics of the Islamic Republic and served as the commander-in-chief of the armed forces.
No-one really knows who might take over that power in the long term, especially as the US and Israeli strikes continue.
During the lead-up to the air strikes, US Secretary of State Marco Rubio stated it plainly:
"I don't think anyone can give you a simple answer as to what happens next in Iran if the supreme leader and the regime were to fall, other than the hope that there would be some ability to have somebody within their systems that you could work towards a similar transition."
The constitutional route — replenish the regime
Iran's constitution outlines a clear process for what happens in the event of the supreme leader's death.
And those processes are now underway.
A temporary three-person council has taken over control of the country — the president, the head of the judiciary and an appointee from the Guardian Council, which is a 12-member body that ensures Iran's rule remains consistent with Islamic criteria and the constitution.
Middle East analyst Amin Saikal from the Australian National University says the three people on the council are "totally loyal to the regime" and some could be potential candidates for the supreme leader position.
"They have already announced that they will follow the path of the deceased Ayatollah Ali Khamenei," he says.
But the three-panel council only leads the nation until the next supreme leader is chosen by the “Assembly of Experts”, a separate 88-person group of clerics who are elected every eight years.
"The assembly is quite factionalised," Saikal says. "There will be a lot of horse trading and there is going to be a lot of compromise."
The Iranian foreign minister has reportedly said the next supreme leader will be named within days, but Ali Vaez, the Iran director with International Crisis Group, is sceptical.
"My guess is that's not going to happen until the dust on this war settles, and only then they would select the leader," Vaez says.
"If they do so now, they would be painting a target on the back of that person."
Vaez says it's important to understand that while Khamenei was Iran's supreme leader, ultimately, there were clear measures in place to maintain the regime in case of his death.
"It was a system and it remains a system — and as long as that system is in place, it is very unlikely that it would become democratic in the true sense of the word.
"It might become more pragmatic, it might become more pluralistic, but not necessarily democratic.
"Although Ayatollah Khamenei was at the pinnacle of power, one has to understand this was not a one-man system."
The military option — force fills the gaps
With the leader gone and the country at war, it's unsurprising that some have speculated about the military overthrowing the regime.
Vaez says that in recent days, power has gravitated towards two men, both of whom are former commanders of Iran's Revolutionary Guards, one of the most powerful and feared organisations in Iran — and the one responsible for brutally suppressing protests in January.
"This is one area where I think Ayatollah Khamenei's worldview outlives him — and that's the fact that you never compromise under pressure because it only invites more pressure," Vaez says.
“Power has gravitated towards two individuals — the Supreme National Council Secretary Ali Larijani and the Speaker of the Parliament Mohammad Bagher Ghalibaf. They’re both former commanders of the revolutionary guards and share Ayatollah Khamenei’s worldview that if you start giving an inch to the population when you're under pressure, they will ask for a mile."
Whether a war footing is enough to destabilise deeply entrenched clericalism in Iran remains to be seen.
Amin Saikal says a military overthrow is a possibility, but he believes it's unlikely.
Instead, he expects the military will continue to protect the theocratic rule that's existed in Iran for 47 years.
"All the indications point to the fact that the military, or particularly the Islamic Revolutionary Guard Corps, at this point is not really interested in taking over.
“They are integrated into the system, and therefore it acts as the guardian of the Islamic regime in terms of not only defending it from any internal threats, but also defending it from any external assault.
“Their fortunes are tied to the survival of the regime, because if the regime goes down, then I think they will go down with it. And therefore, it is imperative for them to continue to fight for the survival of the regime and therefore for their own survival."
A public uprising to seize power?
US President Donald Trump has encouraged the public to "rise up" and overthrow the Iranian government, with the backing of US and Israeli airstrikes.
But what would this look like exactly?
It's difficult to see how it could happen without some kind of military backing on the ground, Amin Saikal says.
"The only way they could achieve regime change is to put boots on the ground in Iran and that's not something that the United States will do."
The public may have more of a chance of a successful uprising if security forces defected, according to Saikal, but he says that's also not likely while the country is at war.
"If there are going to be cracks in the military and security forces, then I think the chances of the public trying to prevail, that seems, you know, stronger.
"But let's not really forget that the Iranian public are very much polarised — there are those who are opposed to the regime and there are those who also strongly supported the regime."
Ali Vaez says that even if the US and Israel continue to put significant cracks in the Iranian regime from the air, it's difficult to see how opponents could take down the government.
"If the remnants of the regime survive, they can still terrorise the population with small arms. Let's remember that last month they committed a massacre against their own people, killing several thousand Iranian protesters with small arms, not with tanks and fighter jets," Vaez says.
"That kind of capacity is hard to eliminate without boots on the ground.
"It's hard to imagine that what President Trump is seeking, which is that the Iranian people will rise from below and finish the job he has started from above, would actually materialise."
Amin Saikal believes a long war is now likely in Iran, despite what Donald Trump and Israeli Prime Minister Benjamin Netanyahu might want.
“The ultimate goal of the regime is survival and it wants to endure," he says.
"And for that reason, one could expect the regime to fight with the last drop of its blood. And the Iranian leaders have made that very clear.
"I doubt it very seriously that the killing of the Iranian supreme leader, Ayatollah Ali Khamenei, will make any fundamental difference. It is certainly a blow to the regime, but it's not an insurmountable problem."
