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The crew of a Ukrainian jetliner that crashed in Iran never made a radio call for help and were trying to turn back to the airport when the plane went down, according to an initial Iranian report released Thursday on the crash that killed 176 people.

Meanwhile, Ukrainian President Volodymyr Zelenskiy pledged to discover the "truth" behind the crash, and announced investigators from his country had arrived in Iran to assist in the probe.

The Iranian report suggested a sudden emergency struck the Boeing 737 operated by Ukrainian International Airlines early Wednesday morning, when it went down just moments after taking off from the Imam Khomeini International Airport in Tehran.

Investigators from Iran's Civil Aviation Organization offered no immediate explanation for the disaster, however.

Eyewitnesses, including the crew of another passing flight, described seeing the plane engulfed in flames before crashing, the report said.

The crash caused a massive explosion, likely because the aircraft had been fully loaded with fuel for the flight to Kyiv, Ukraine.

Both of the so-called “black boxes” containing data and cockpit communications from the plane had been recovered, though they had been damaged, the report added.

President Volodymyr Zelensky places flowers at a memorial for the plane crash victims at the Boryspil airport outside Kiev on Thursday. AFP - Getty Images

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Zelenskiy also said he planned to call Iranian President Hassan Rouhani about the crash and the investigation.

“Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash,” he said. “We will surely find out the truth.”

Zelenskiy said the crash will be investigated by a committee created by Iran's civil aviation agency. He also cautioned against speculation and conspiracy theories while the investigation is ongoing.

"I call on the international community, including Canada, to join the investigation," he added.

Ukrainian officials, for their part, initially agreed with Iranian suspicions that the 3½-year-old plane was brought down by mechanical trouble, but later backed away from that and declined to offer a cause while the investigation is ongoing.

A top Ukrainian security official said in a Facebook post Thursday that four main scenarios of what could have happened are being looked into, including an anti-aircraft missile strike, a collision with a UAV or unmanned aerial vehicle, an engine failure and an explosion inside the aircraft as the result of a terrorist act.

The plane was carrying 167 passengers and nine crew members from several countries, including 82 Iranians, at least 63 Canadians and 11 Ukrainians, according to officials.

The crash just before dawn scattered flaming debris and passengers’ belongings across a wide stretch of farmland. It also came shortly after Iran launched a ballistic missile attack against Iraqi military bases housing U.S. troops amid a confrontation with Washington over the killing of a top Iranian general in a drone strike last week.

A child's shoe at the crash scene of a Ukrainian airliner near Imam Khomeini airport near Tehran on Wednesday. Borna Ghassemi / AFP - Getty Images

Many of the passengers were believed to be international students attending universities in Canada; they were making their way back to Toronto by way of Kyiv after visiting their families during the winter break.

The flight also included a family of four and newlyweds. The passenger list included several teenagers and children, some as young as 1 or 2.

“Know that all Canadians are grieving with you,” Canadian Prime Minister Justin Trudeau said, addressing the victims’ families.

Jan. 9 was declared a national day of mourning in Ukraine.

The disaster could further damage Boeing’s reputation, which has been battered by the furor over two deadly crashes involving a different model of the Boeing jet, the much-newer 737 Max, which has been grounded for nearly 10 months.

Boeing extended condolences to the victims’ families and said it stands ready to assist.

Senior U.S. investigative sources said Wednesday Boeing and U.S. government investigators likely won't be able to investigate the crash.