All 176 people on board were killed when a Ukraine International Airlines plane crashed in Iran early on Wednesday morning.
Authorities in Iran and Ukraine, as well as at the airline, have offered statements and press conferences, but a number of key unanswered questions are still swirling.
Investigations have kicked off amid rampant speculation that current political tensions between the US and Iran could have contributed to the plane crash.
Information about who was on the passenger jet — Flight PS 752 — and what happened before the crash remain lacking.
Here are the unanswered questions.
What happened on the flight?
A complete timeline of the flight is yet to emerge.
But we don't know exactly what time it crashed — just that it was only in the air for a few minutes, based on flight-tracking software, and that the debris was found about six miles from the airport from where it took off.
Authorities also said the plane burst into flames shortly after takeoff, but whether the plane was already on fire before it crashed to the ground is not yet clear.
A video shared by the partially state-run Iranian Students' News Agency appears to show the plane on fire in the air before hitting the ground and filling the sky with flames, but the video's content and connection to this crash has not yet been verified.
Who was killed?
Ukraine's foreign minister said that the victims mostly came from Iran and Canada.
Vadym Prystaiko said the victims were 82 Iranians, 63 Canadians, and 11 Ukrainians, as well as 11 Swedish citizens, four Afghan citizens, three UK citizens, and three German citizens. Nine of those on board were crew members.
But he did not identify the victims.
Sky News identified the three UK citizens on board, while the airline identified the pilots as Volodymyr Gaponenko, Alexei Naumkin, and Sergey Khomenko. All had a minimum of 7,600 hours on Boeing 737 planes. Vice also identified some Canadian victims.
Ukraine International Airlines said it will post the passenger list on its website "after final confirmation of their presence on board of the aircraft."
Was the plane shot down?
Some aviation experts have argued that the plane was likely shot down; others have said it was too early to speculate about the cause.
But the idea that the plane was deliberately downed, including shot down by a missile, is speculation at the moment, and its account is contradicted by authorities.
Ukraine's president, Volodymyr Zelensky, warned against "speculation or unchecked theories regarding the catastrophe" until official investigations were done. He said, "Our priority is to establish the truth and those responsible for this terrible catastrophe."
Iranian authorities said in the hours after the crash that it had been caused by technical problems, dismissing the idea that it could have been a terrorist or military attack.
President Volodymyr Zelensky.
Qassem Biniaz, an official at the Iranian Ministry of Roads and Urban Development, told state news agency IRNA that an engine caught fire and the pilot was unable to regain control, The New York Times reported.
Meanwhile, Ukrainian Prime Minister Oleksiy Honcharuk refused to rule out the idea that the plane could be downed by a missile, but cautioned against speculation before the investigation.
CFM, the French-American maker of the jet engine, said that any speculation on causes was premature, according to Reuters.
"We have no further information at this time. Any speculation regarding the cause is premature," the company said.
Ukraine's embassy in Tehran initially dismissed the idea of terrorism or a rocket attack soon after the crash, blaming an engine failure instead. But that statement was later replaced by one that says the cause is unknown and is being investigated.
According to Reuters, the embassy said the earlier statement was based on preliminary information but was not official, and that Iranian authorities had asked the embassy to remove it.
Hours before the plane crash, Iran attacked two Iraqi military bases that housed US and coalition forces with ballistic missiles. There is no evidence that the two incidents are linked.
Were there any faults with the plane?
Ukraine International Airlines has sought to distance itself from the possibility that there was a fault with the plane or its crew.
It said in the hours after the crash that the plane was built in 2016 and that it had last completed maintenance checks just two days ago.
It also said that the plane was one of the best in its fleet and had an experienced crew. The airline had never had a fatal flight before.
The airline's vice president of operations, Ihor Sosnovsky, said the airline doubted the crew had made mistakes: "Given the crew's experience, error probability is minimal."
The plane model, the Boeing 737-800 NG, has been in the air since the 1990s, and is considered the most popular aircraft in use today. It has been involved in some crashes in the past, though no recent crashes have been attributed the plane's design.
The crash may ramp up pressure for Boeing as it deals with the fallout of two fatal crashes by two 737 Max planes in 2018 and 2019, both of which were believed to be caused by a flawed flight-control system.
But the 737 model involved in Wednesday's crash does not use the same software believed to have played in a role in those doomed flights.
Boeing said in a Wednesday statement: "This is a tragic event and our heartfelt thoughts are with the crew, passengers, and their families. We are in contact with our airline customer and stand by them in this difficult time. We are ready to assist in any way needed."
How will the investigations work?
Under international rules, Iran must investigate the crash, though typically a number of different investigations take place into plane crashes.
Ukraine's President Zelensky also said that he had instructed his prosecutor-general to open criminal proceedings after the crash.
Ukraine International Airlines said it would take "all measures" to determine the cause of the crash, and that Ukraine, Iran, and Boeing representatives would also be involved.
But Boeing, a major American company with close ties to the US government, may face problems in getting involved with the investigation because of US sanctions on Iran and newly heightened tensions between the two countries.
Mehr, a semi-official Iranian news agency, quoted the head of Iran's civil aviation authority as saying the country would not give the plane's black boxes to Boeing, Reuters reported. He said he was not sure what country Iran would give them to.
It is not clear what the role the independent US National Transportation Safety Board, which investigates plane crashes, will play. It said it is "monitoring the developments" and is working with US agencies to "determine the best course of action."
Investigators have not yet released a timeline for when they expect to release any preliminary conclusions. Final reports usually take months to complete.
This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.
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