In this handout photo provided by the Ukrainian Presidential Press Office, the plane carrying Ukrainian experts prepares to…
In this official Ukrainian photo, a plane with Ukrainian experts prepares to leave for Tehran at the Boryspil airport outside Kyiv, Ukraine, Jan. 8, 2020. Questions have arisen about who will investigate the crash of a Ukrainian plane in Iran.

Authorities in Iran are investigating why a Ukrainian passenger jet crashed minutes after takeoff from Tehran's airport early Wednesday, killing all 176 people on board.

The Ukraine International Airlines (UIA) flight was bound for Kyiv with 167 passengers and nine crew members. Ukrainian Foreign Minister Vadym Prystaiko said the dead included 82 Iranians and 63 Canadians along with Ukrainians, Swedes, Afghans, Germans and Britons.

The flag over the Canadian Parliament in Ottawa was lowered to half-staff Wednesday. Prime Minister Justin Trudeau said the entire country was "shocked and saddened" at one of its worst losses of life in a single day in years.

Trudeau said 138 of the passengers had planned to take an onward flight from Kyiv to Toronto, many of them Iranian students hoping to return to school after a winter break with their families in Iran. He promised to work for a thorough investigation of the crash.

U.N. Secretary-General Antonio Guterres offered his condolences, through a spokesman, to the families of the victims and the various countries from which they came. 

Flowers and candles are placed in front of portraits of crew members of the Ukraine International Airlines plane that crashed in Iran, at a memorial at the Boryspil International Airport outside Kyiv, Jan. 8, 2020.

U.S. Secretary of State Mike Pompeo also issued a statement of condolence and said Washington was prepared to offer Ukraine "all possible assistance." He said the U.S. also called for "complete cooperation with any investigation" into the cause of the crash.

Iranian state television reported that both the black box voice and data recorders from the Boeing 737 aircraft had been recovered from the crash site, a swath of farmland on the outskirts of the Iranian capital.

Iran's semiofficial Mehr News Agency quoted the head of the nation's civil aviation agency as saying he did not know which country would get the black boxes for analysis, but that Iran would not hand them over to U.S.-based Boeing, the aircraft's manufacturer.

WATCH: Ukraine, Canada Demand Thorough Investigation of Boeing Crash in Iran

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Usual NTSB role

The U.S. National Transportation Safety Board typically participates in investigations of overseas air crashes when a U.S. airline or plane manufacturer is involved. But given the heightened tensions between Washington and Tehran, and the fact that the two sides have no diplomatic relations, it was uncertain whether the NTSB would be involved in the investigation of the UIA crash.

In a statement sent to VOA Ukrainian, the NTSB said it was "monitoring developments surrounding the crash of UIA Flight 752" and was "following its standard procedures" for international aviation accident investigations.

"As part of its usual procedures, the NTSB is working with the State Department and other agencies to determine the best course of action," it said.

"The U.S. has not participated in an accident investigation in Iran since Iran's 1979 Islamic Revolution. So it is very unlikely that the NTSB will be involved," said Madhu Unnikrishnan, editor of U.S. airline news service Skift Airline Weekly in a VOA Ukrainian interview.

Passengers' belongings are pictured at the site where a Ukraine International Airlines plane crashed after takeoff from Iran's Imam Khomeini Airport, on the outskirts of Tehran, Iran, Jan. 8, 2020.

The Convention on International Civil Aviation, to which Iran is a signatory, does not require Tehran to hand over the data recorders to the NTSB or Boeing, said Andriy Guck, a Ukraine-based attorney and aviation expert.

"There is a duty to investigate," Guck told VOA Ukrainian in a phone conversation. "Iran can decide to investigate the black boxes by itself or transfer them to a foreign laboratory. But if the Iranians do not allow anyone else to participate in the examination of the boxes, it will raise doubts about their investigation."

Editor Unnikrishnan said, "The key will be to involve other European countries that have experience in these matters in the analysis of the black boxes."

"If Iran sends them to Europe, I think the analysis will be trusted internationally," he said.

The fiery crash occurred hours after Iran launched a ballistic missile attack on Iraqi bases housing U.S. soldiers in response to last week's U.S. drone attack that killed Iranian Quds Force commander Qassem Soleimani.

Iranian officials said the crash happened because of a mechanical failure. Ukrainian officials initially agreed with that assessment, but then backed away and declined to offer a possible cause while the investigation was ongoing.

Insufficient information?

Some U.S. aviation experts said they could not see how Iran could possibly know that a mechanical problem caused the crash without looking at the flight data recorders or examining the engines.

Iranian officials dismissed speculation that a missile brought down the plane.

Ukraine President Volodymyr Zelenskiy urged the public "to refrain from ... expressing uncorroborated theories." He ordered a sweeping inspection of all civil airplanes in Ukraine "no matter the conclusions about the crash in Iran."

The Iranian road and transportation ministry said it appeared that one of the plane's two engines caught fire, with the pilot then losing control of the jetliner. The flight-tracking website Flightradar24 said the plane never got above 2,500 meters (8,200 feet).

An Iranian investigator said it appeared the pilot was unable to communicate with air traffic controllers in the moments before the crash. Videos taken immediately after the crash showed fires lighting up the darkened fields in the predawn hours. 

Ukraine International Airlines President Yevhen Dykhne attends a briefing at Boryspil International Airport outside Kyiv, Ukraine, Jan. 8, 2020.

Ukraine International Airline President Yevhen Dykhne said, "It was one of the best planes we had, with an amazing, reliable crew."

The jet was built in 2016. It was a Boeing 737-800 model, a commonly used commercial jet with a single-aisle cabin that is flown by airlines throughout the world. It is an older model than the Boeing 737 Max, which has been grounded for nearly 10 months following two deadly crashes.

Tatiana Vorozhko of VOA's Ukrainian service and Michael Lipin of VOA Persian contributed to this report.