Speculation naturally ran wild when a passenger plane crashed in Tehran shortly after missiles had been fired by Iran at a base housing US troops in Iraq on Wednesday.

Iran quickly attributed the crash to mechanical faults and a pilot losing control, but there was early speculation from some experts that the plane could have been shot down.

Days later, the US, Canada and Australia said they had intelligence indicating that was the case and, after initial denials, Iran eventually admitted the plane was "unintentionally" shot down.

The crash claimed the lives of 176 people and, with victims from seven countries, a number of governments are demanding answers.

So, what do we know and who's saying what?

Where was the flight heading before it was shot down in Iran?

All of the 176 people on the flight died. ( AP: Mohammad Nasiri )

The plane that was brought down was a Ukraine International Airlines Boeing 737-800, leaving Tehran's Imam Khomeini International Airport bound for Kiev's Boryspil International Airport.

Flight PS752 got off the ground just after 6:00am local time and, as we know, never landed — exploding before it reached 8,000 feet, according to FlightRadar24, and crashing to the ground near Shahedshahr, on the outskirts of the Iranian capital.

The Boeing 737-800 crashed on the outskirts of Tehran. ( Supplied: FlightRadar24 )

It has been reported that the plane had turned around before it went down.

FlightRadar24 measured the time between take-off and the crash as about six minutes.

How do people say the plane crashed?

Initially, two tales were being told — one by Iran, and one by the US, Canada, Australia and other allies.

Fears of World War III breaking out were (tentatively) calmed in the immediate aftermath of the crash, when Iran said initial investigations suggested a mechanical problem caused the plane to burst into flames in midair.

But days later a US official, followed by the prime ministers of Canada and Australia, said they had intelligence that the plane was hit by an Iranian missile.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 41 seconds 41 s Canandian Prime Minister Justin Trudeau says evidence suggest Ukrainian plane was shot down by Iranian air missile

On Friday, both Justin Trudeau and Scott Morrison said the intelligence suggested the shooting-down of the plane was likely unintentional.

US officials said the plane was hit by a SA-15 missile, which is a Russian-made missile deployed as part of the Tor air defence system — basically, missiles fired from the back of a tank — designed for short-range, surface-to-air launches (eliciting memories of the Russian Buk missile that took down MH17).

One US official said US satellites had detected the launch of two missiles shortly before the plane crashed, followed by evidence of an explosion. Two officials said Washington believed the downing of the plane was accidental.

Ukrainian President Volodymyr Zelenskiy said in the immediate aftermath that the investigation would look at several possible causes. ( AP: Ukrainian Presidential Press Office )

Iran's Government denied the accusations, with spokesman Ali Rabiei on Friday describing the reports as "psychological warfare against Iran", while the head of the country's Civil Aviation Organisation, Ali Abedzadeh, said the missile claims were "illogical rumours" and "scientifically impossible".

A day later, the Iranian Government admitted the plane was "unintentionally" shot down, blaming human error. Iran's Foreign Minister also criticised "US adventurism".

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Ukrainian President Volodymyr Zelenskiy said the Government was considering several possible causes of the crash and was pushing the investigation forward.

Regarding the investigation, Mr Abedzadeh said the black box for the flight would "not be handed over to the Americans" when asked if the black box would be returned to the country of manufacture.

It was later reported that the US National Transportation Safety Board would take part in the investigation.

What did people in Iran see?

It's 2020, so it shouldn't be surprising that the last moments of flight PS752 were reportedly caught on camera.

Not long after the crash, footage emerged appearing to refute Iran's early claims of a midair fault or pilot error, with video recorded from the ground showing a bright object moving through the sky before an explosion.

The explosion appeared to be close to PS752's last transponder position.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 19 seconds 19 s Footage shows a midair explosion close to Ukraine International Airlines flight PS752's last transponder position

Other verified footage showed the plane on fire and slowly going down before exploding in a bright flash as it hits the ground.

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Why were so many Canadians on a flight from Tehran to Kiev?

Communities across Canada have gathered to mourn those killed in Iran. ( AP: Chris Young via The Canadian Press )

There were 176 people on board — including 82 Iranians, 57 Canadians, 11 Ukrainians, 10 Swedes, four Afghans, three Germans and three Britons, according to Ukraine's Foreign Minister — all of whom died.

Iran identified 147 passengers as Iranian, but it is believed many passengers were dual nationals.

More than 200,000 Canadian citizens are of Iranian descent and the country is a popular destination for students doing research abroad, but there is no direct flight from Iran to Canada, making the route through Kiev one of the cheapest options available.

It has been reported that many of the Canadian victims were students returning from a trip to Iran.

Mr Trudeau said 138 of the passengers were scheduled to make the connection to Canada after landing in Kiev and that connecting flight landed in Toronto with many empty seats.

Canadian Foreign Minister Francois-Philippe Champagne said it was an "extremely fluid" situation and the number of Canadians could change as more came to light regarding passengers carrying dual citizenship.

The Canadian Government initially said 63 citizens died in the crash before that number was lowered to 57.

Has this sort of thing happened before?

Considering the investigation is ongoing, we're yet to receive the black-box data and there are multiple stories being told, it's hard to say if there is an exact parallel.

But there are some common threads with the case of Iran Air flight 655, which is clearly etched in the mind of Iranian President Hassan Rouhani, who tweeted about it before tensions escalated on Wednesday.

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Flight 655 was shot down in 1988 by the USS Vincennes, an American guided-missile cruiser. There were 290 passengers and crew aboard the Airbus and they were all killed.

But on that occasion, both the Iranians and the Americans acknowledged that the US ship had brought down the plane, with the argument over why and how severe the punishment should be.

The captain of the USS Vincennes was honourably discharged three years after the tragedy. ( Reuters: Spiros Mantzarlis )

The American account was that the crew had mistaken the passenger jet for an Iranian F-14 warplane, and was unable to contact the plane as communications got confused during the fog of a fight against Iranian gunboats.

Iran argued that it was an intentional act by the US military, which was operating in Iranian waters at the time.

Then-vice-president George HW Bush defended the actions at the United Nations and eventually, a resolution expressing "deep distress" and " sincere condolences" was ratified by the UN.

The US eventually agreed to pay US$61.8 million to the victims' families.