The crew of a Ukrainian jetliner that crashed in Iran never made a radio call for help and were trying to turn back for 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 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 when the plane hit the ground, 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 and some parts of their memory was lost, the report added.

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,” Zelenskiy 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.

Ukrainian officials, for their part, initially agreed with Iranian suspicions that the three-and-a-half-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.

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 it killing a top Iranian general in a drone strike last week.

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 with family during the winter break.

Canadian Prime Minister Justin Trudeau said 138 of the passengers were bound for Canada. 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,” Trudeau said, addressing the victims’ families.

Ukraine’s president called for Canada to be involved in the investigation.

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.