The crew of a Ukrainian jetliner that crashed in Iran did not make 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 disaster that killed 176 people.

The 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.

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.

Black boxes damaged

The report also confirmed that both of the so-called “black boxes” that contain data and cockpit communications from the plane had been recovered, though they had damaged and some parts of their memory was lost. It also said that investigators have initially ruled out laser or electromagnetic interference as causing the crash.

Iran's aviation authority has said it will not hand over the black boxes to Boeing.

The head of the authority, Ali Abedzadeh said: “We will not give the black boxes to the manufacturer and the Americans. It’s not yet clear which country the black box will go to for the investigation."

He added that Iran will be leading the investigations and Ukrainians could be present throughout.

Ukrainian President Volodymyr Zelenskiy, meanwhile, said crash investigators from his country had arrived in Iran to assist in the probe. He 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.”

Canada is offering technical assistance to the upcoming investigation in Iran. Canada's Transportation Safety Board said it has appointed an expert to monitor the progress of the investigation by Iran's civilian aviation agency.

Aviation experts were sceptical about Iran’s initial claim that the plane was brought down by a mechanical problem.

“Something unusual happened," Garneau said.

Canada and Ukraine want answers

Zelenskiy and Canada's Justin Trudeau have vowed to get answers for the crash. Zelenskiy cut short a visit to Oman. On Facebook, he posted: "Our priority is to find the truth and everyone responsible for the tragedy."

Out of the 176 people who died in the crash, 82 were Iranian, 63 from Canada, 11 from Ukraine, 10 from Sweden, four from Afghanistan, three from Germany and three from the UK.

Canadian Prime Minister Justin Trudeau said Wednesday 138 passengers on the flight were connecting to Canada. The flight included many international students who were studying at universities across Canada. Newlyweds and a Canadian family of four were also on the flight.

Trudeau said his government is pushing to be part of the Iranian-led investigation of the plane crash near Tehran that killed everyone on board. Getting answers from Iran might prove difficult as Canada closed its embassy in Iran in 2012 and suspended diplomatic relations.

'Canadians have questions and they deserve answers'

The crash of the Ukraine International Airlines plane came hours after Iran launched a ballistic missile attack on Iraqi bases housing U.S. soldiers, but Iranian officials said they suspected a mechanical issue brought down the 3½-year-old Boeing 737-800 aircraft. Ukrainian officials initially agreed, but later backed away and declined to offer a cause while the investigation is ongoing.

“Canadians have questions and they deserve answers," Trudeau said.

Asked if he could say whether the plane was shot down or not, Trudeau said: “I cannot. It's too early to speculate.”

“Know that all Canadians are grieving with you," he said, addressing the victims' families.

Transport Minister Marc Garneau said soon after the departure of the plane contact was lost. He said the black box information is needed.

One of the worst losses of life for Canadians in aviation disaster

In 1985 a bomb exploded and killed 329 people aboard an Air India flight. Air India Flight 182 from Montreal to New Delhi exploded over the Atlantic Ocean near Great Britain on June 23, 1985. Most of the victims were Canadian.

The Tehran to Toronto route via Kyiv is an affordable route for Iranian Canadians and international students. There are no direct flights.

Most victims were visiting family

Payman Paseyan, a member of the Iranian-Canadian community in Edmonton, Alberta, said about 27 people from Edmonton, including a family of four that he knew, were on the flight. Mourners gathered outside Alberta's legislature on Wednesday night for a vigil.

Two professors from the University of Alberta, Pedram Mousavibafrooei and Mojgan Daneshmand, and their daughters Daria and Dorina died. Paseyan said he often would go to the gym with the father and described him as a nice guy who often visited his former restaurant with his family.

He said most of the victims were visiting family in Iran over the holidays and that many were dual citizens and many were international students. “One of the reasons why you take that flight is you wouldn't want to take a flight that has a connection in the United States because international students can't do that," he said.

Younes Zangiabadi, a board member of the Iranian Canadian Congress, said because of U.S. sanctions, there are not many options for Iranian-Canadians to travel to Iran

Paseyan said members of the Iranian-Canadian community learned of the crash while being glued to the news after Tuesday's missile attacks in Iraq.

He said there are questions about what caused the crash but said that's not the focus right now.

“Whether it's a missile, mishap or a technical issue or whatever, we want our community members. We lost one per cent of our Edmonton Iranian community members on that flight. It's just terrible," he said.

Many international students were confirmed among the crash victims — four from Western University, three from the University of Ottawa, and two from the University of Guelph. The University of Toronto said the names of six of their students were on the flight manifest.

Western University said three current graduate students died and one was an incoming graduate student. They did not name the students. The University of Waterloo also said two PhD students' names were on a list of passengers provided by the airline.

In a letter to parents, the principal of Northern Secondary School says the school is grieving the loss of Maya Zibaie, a Grade 10 student.

A dental office in Aurora, Ontario confirmed that Parisa Eghbalian, a dentist, and her daughter Reera Esmaeilion died. Eghbalian's husband, Hamed Esmaeilion, is also a dentist at E&E Dentistry, but was not travelling with his wife and child. He was supposed to pick up his 9-year-old daughter and wife at Toronto's airport. Now he's headed to Iran.

Uncertainty about the causes

In a statement, Ukraine International Airlines said that flight PS752 "disappeared from the radars a few minutes after departure from Tehran International Airport."

The plane had taken off when a fire struck one of its engines, said Qassem Biniaz, a spokesman for Iran's Road and Transportation authority.

The pilot of the aircraft then lost control of the plane, sending it crashing into the ground, Biniaz said, according to the state-run IRNA news agency.

Throughout the day, numerous theories about the causes of the plane crash emerged on social media.

In the immediate aftermath, Ukraine's embassy in Iran issued a statement in which it ruled out terrorism or a rocket attack. However, that statement was later removed and then replaced one that said it was too early to draw conclusions.

In a Facebook post, Zelensky wrote: "All possible versions must be examined."

The timing of the crash raised immediate suspicions, as it comes at a time of high tensions between the US and Iran. On the same day, Iran had retaliated against the US killing of one of its top commanders by firing missiles towards US-American troops in Iraq.

It can take longer than a year for investigations to come to any conclusions. Given the US-Iranian tensions and the fact that Canada has no diplomatic relations with Iran at the moment, it may be even harder to facilitate communication between all the involved parties.