Iran plane crash: What we know about flight PS752
The crash is the centre of an investigation that has now become intertwined with escalating tensions between the US and Iran.
Here is what we know.
On 8 January, at 06:12 local time (02:42 GMT), UIA flight PS752 took off from Tehran's Imam Khomeini International Airport.
The plane was a Boeing 737-800 - one of the international airline industry's most widely used aircraft models.
Before it had left the airport's air space, the plane turned around and tried to return to the runway. Shortly afterwards, it crashed.
Who was on board?
Of the 176 people on board, 15 were children.
Among the victims were 82 Iranians, 63 Canadians, 11 Ukrainians including all nine crew, 10 Swedes, four Afghans, three Britons and three Germans, Ukrainian Foreign Minister Vadym Prystaiko said.
But the German government later said "we currently have no knowledge that German citizens are among the victims of the plane crash in Iran".
And UK Prime Minister Boris Johnson said on Thursday that four British nationals were on board.
Iran's head of emergency operations said 147 of the victims were Iranian. That would suggest that 65 of the foreign nationals had dual nationalities.
How is it being investigated?
Under international protocol, the country where the plane crashes usually leads the investigation.
As the aircraft was made in the US, US officials, including from its National Transportation Safety Board (NTSB), would typically participate in any inquiry.
Iran had initially ruled out handing over any information to the US authorities. But the country's representative at the UN's International Civil Aviation Organization told Reuters on Thursday that Iran had formally invited the NTSB to take part in the investigation, and it has agreed to assign an investigator.
Boeing, the plane's manufacturer, has said it is ready to assist in the investigation and will support the NTSB.
The aircraft's "black boxes", which record flight data and sound within the cockpit, were recovered from the wreckage.
What caused the crash?
Iranian authorities have blamed technical issues, but the crash's timing - just hours after Iran launched missiles at US targets in Iraq - provoked speculation about other possible causes.
Tom Burridge, the BBC's transport correspondent, said the rapid disappearance of tracking data suggested a catastrophic incident occurred.
Some aviation experts have also cast doubt on claims, made shortly after the crash on Iranian state media, that the crash was likely to have been caused by an engine fire.
Commercial aircraft are designed to be able to withstand - in general - a failed engine and to land safety.
On Thursday, Canadian Prime Minister Justin Trudeau said evidence suggested an Iranian missile brought down the aircraft by accident.
"We have intelligence from multiple sources, including our allies and our own intelligence," Mr Trudeau told a news conference in Ottawa. "The evidence indicates that the plane was shot down by an Iranian surface-to-air missile. This may well have been unintentional."
He was echoing earlier reports in US media, which said Pentagon officials were confident that the aircraft was shot down.
Newsweek cited Pentagon and Iraqi sources as saying the strike was probably accidental. CBS News then said US intelligence officials had picked up signals indicating a radar was turned on and two missiles launched.
A team of 45 Ukrainian experts arrived in Tehran on Thursday morning to assist Iranian authorities and help with the identification and repatriation of victims.
Oleksiy Danylov, head of Ukraine's national security council, said that four main possible crash causes were being investigated:
- a missile strike
- a mid-air collision with a drone or other flying object
- engine destruction/explosion due to technical reasons
- an explosion inside the plane as a result of a terror attack
President Volodymyr Zelensky has asked people to refrain from speculation regarding the crash and insisted "a thorough and independent investigation will be conducted".
How has Iran responded?
The government in Tehran has ruled out a missile strike by its air defences.
Iran's Civil Aviation Organisation (CAOI) released its own initial report into the crash on Thursday.
It said the Boeing 737-800 suffered a technical problem shortly after take-off, and cited witnesses, including the crew of another passenger plane, that it was on fire prior to impact.
Authorities said they lost radar contact when the plane was at an altitude of about 8,000ft (2,400m), minutes after taking off.
No radio distress call was made by the pilot, the report said.
"Several domestic and foreign flights were flying in Iranian space at the same altitude. The issue of the missile's impact on the aircraft cannot be true in any way," CAOI chief Ali Abedzadeh said.
He added that Iran would not hand over the plane's black boxes to Boeing or to US authorities.
What was the plane's safety record?
UIA said the aircraft was manufactured in 2016 and last underwent scheduled maintenance on Monday. It added that there were two pilots and an instructor on board, all of whom were experienced.
Each had spent between 7,600 and 12,000 hours of flight-time on the 737-800 prior to the crash, the airline said.
"Given the crew's experience, error probability is minimal. We do not even consider such a chance," UIA's vice-president Ihor Sosnovsky said.
Founded in 1992, the airline had never suffered a fatal accident before Wednesday.
Flight PS752 was a 737-800, the most popular made by Boeing, with almost 5,000 produced since its 1994 launch.
It is distinct from Boeing's newer 737 Max model, which is currently grounded over safety concerns, and does not have the automated system that is thought to have contributed to two deadly crashes involving the Max jets.
The 737-800 is generally considered to have a good safety record. Aviation Safety Network, a flight-safety tracking group, say Wednesday's crash is the eighth fatal incident involving the model.
Harro Ranter, the group's head, told the Wall Street Journal that pilot error was found as the likely cause in all but one of the previous accidents.