Relatives of the flight crew members of the Ukraine International Airlines Boeing 737-800 plane that crashed in Iran, mourn at…
Relatives of the flight crew of the Ukraine International Airlines Boeing 737-800 plane that crashed in Iran, mourn at a memorial at the Boryspil International Airport outside Kyiv, Ukraine, Jan. 8, 2020.

Iranian investigators said Thursday the crew of a Ukrainian passenger jet that crashed shortly after taking off from Tehran’s airport had tried to turn back, and that the pilot made no radio communications about any problems.

The initial report from Iran’s Civil Aviation Organization also cited witnesses on the ground and in a passing aircraft as saying the Ukraine International Airlines plane was on fire before it hit the ground.

FILE - Debris from a Ukraine International Airlines plane that crashed after taking off from Iran's Imam Khomeini airport, is seen on the outskirts of Tehran, Iran, Jan. 8, 2020.

Ukrainian President Volodymyr Zelenskiy declared Thursday a day of mourning for the 167 passengers and nine crew members who died when the plane bound for Kyiv crashed early Wednesday.

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 fly on 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.

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

In this photo from the Ukrainian Presidential Press Office, the plane carrying Ukrainian experts prepares to depart for Tehran at Borispil international airport outside Kyiv, Ukraine, Jan. 8, 2020.

Data recorders found

Iranian investigators said the voice and data recorders from the Boeing 737 aircraft were recovered from the crash site, a swathe of farmland on the outskirts of the Iranian capital.

Iran’s semi-official 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.

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.

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

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, Andriy Guck, a Ukraine-based attorney and aviation expert, said.

“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,” he said.

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.

Cause of crash not yet known

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.

Some U.S. aviation experts said they cannot 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.

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.

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 company President Yevhen Dykhne attends a briefing at Boryspil international airport outside Kyiv, Ukraine, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff in Iran.

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.