More than a day after a Ukraine International Airlines plane plunged from the sky, carrying all 176 people on board to their deaths outside Iran's Imam Khomeini International Airport, difficult questions linger about exactly what led to the crash.

Authorities in Kyiv and Tehran have both launched investigations into the deadly incident — and they say they are keeping all possible causes on the table.

Iran's Civil Aviation Organization released a preliminary report finding that Flight 752, a Boeing 737-800, was already on fire prior to the craft hitting the ground, though "no radio messages were received by the pilot regarding unusual circumstances." The report also found that the plane had changed direction midflight, suggesting that it "was on its way back to the airport at the time of the crash."

The Iranian aviation authority noted that the plane's "black box" flight recorders, containing its flight data and cockpit recordings, had been damaged in the crash but nevertheless were recovered and now are under review.

The group offered no final conclusions on the cause of the crash.

President Trump, meanwhile, said Thursday that he harbors his own suspicions about the crash. At a news conference Thursday, the president suggested without evidence that "somebody could have made a mistake."

Ukraine's President Volodymyr Zelenskiy, for his part, vowed to get to the bottom of what happened.

"Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash. We will surely find out the truth," he said in a recorded statement Thursday. "For this purpose, a thorough and independent investigation will be conducted in accordance with international law."

Oleksiy Danilov, secretary of Ukraine's National Security and Defense Council, is helping lead a joint commission in Tehran. According to a Facebook post by Danilov, the commission made up of international specialists is considering a wide range of theories at the moment — including a technical problem with the engine, a collision with another flying object, a terrorist act or a missile strike.

The Ukrainian official said the commission includes experts who investigated the 2014 attack on Malaysia Airlines Flight 17, which internationalinvestigators say was launched from rebel-held territory in eastern Ukraine using a Russian missile.

Among the dead in the crash of Flight 752 were 82 Iranians, 63 Canadians and 11 Ukrainians, including the crew. Passengers from Sweden, Afghanistan, Germany and the U.K. were all killed in the crash as well.

Zelenskiy declared Thursday a national day of mourning for Ukraine, with flags across the country ordered to fly at half-staff. He also spoke with leaders in Sweden, Afghanistan and the U.K. to offer his condolences for their countries' losses.

"Yesterday was without exaggeration a tragic day in the history of our countries," he told British Prime Minister Boris Johnson, asking the U.K. to join its investigation into the incident, according to a summary of the call.

The plane crash came amid a period of heightened tensions between Iran and the U.S., sparked last week by the U.S. airstrike that killed prominent Iranian military commander Qassem Soleimani in Iraq. U.S. authorities say they carried out the drone strike — which also killed a major Iraqi militia leader — to prevent planned violence against American diplomats, not long after a throng of protesters tried to storm the U.S. Embassy in Baghdad.

This week, Iran retaliated for that killing with ballistic missile strikes on Iraqi bases hosting U.S. military forces, in an operation that came within hours of the of the plane crash outside Tehran.

At this point, authorities have not publicly linked the two incidents.

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