A Ukrainian commercial airliner crashed on January 8 soon after taking off from Iran's capital, Tehran, killing all 176 people on board, mostly Iranian nationals, but also Western and Ukrainian citizens.
Debris and smoldering parts from the Boeing 737-800NG belonging to Ukraine International Airlines (UIA) were strewn across a field southwest of the Iranian capital's Imam Khomeini International Airport as rescue teams with face masks retrieved bodies.
The crash came just hours after Iran launched a ballistic-missile attack targeting two bases in Iraq housing U.S. forces in retaliation for the killing of Iranian Revolutionary Guards Corps commander Qasem Soleimani.
The cause of the crash was not immediately clear. The 3-year-old aircraft was en route to Kyiv.
Ukrainian authorities said the victims included 82 Iranians, 63 Canadians, 11 Ukrainians including all nine crew, 10 Swedes, four Afghans, three Britons, and three Germans.
Ukrainian President Volodymyr Zelenskiy said in a statement that he had instructed the country's prosecutor-general to open criminal proceedings over the crash.
Ukraine will send a team of experts to Iran later in the day to investigate the circumstances of the crash, the president said on Facebook.
"Our priority is to establish the truth and those responsible for this terrible catastrophe," Zelenskiy wrote.
Iran and Ukraine agreed "to coordinate further actions of our investigation groups closely to determine the cause of the terrible plane crash," according to Ukrainian Foreign Minister Vadym Prystayko.
In a statement, Canadian Prime Minister Justin Trudeau said his government will "work closely with its international partners to ensure that this crash is thoroughly investigated, and that Canadians' questions are answered."
U.S. Secretary of State Mike Pompeo said the United States was prepared to offer Ukraine "all possible assistance...with any investigation into the cause of the crash."
Ukraine's embassy in Tehran initially blamed engine failure but later removed the statement.
An embassy official said Iranian authorities had asked it to rescind an earlier statement from Iran based on preliminary information.
Iranian TV said the crash was due to unspecified technical problems, and Iranian media quoted a local aviation official as saying the pilot did not declare an emergency.
State media reported that the plane caught fire after crashing, but a video aired by the state broadcaster appeared to show the plane already on fire as it fell from the night sky.
Zelenskiy said the airworthiness of Ukraine's entire civilian fleet will be tested.
"My sincere condolences to the relatives and friends of all passengers and crew," Zelenskiy said, announcing that he was breaking off his visit to Oman to return to Kyiv.
In a separate comment on Facebook, Zelenskiy also warned against speculation about the crash.
"I ask everyone to keep from speculating and putting forth unconfirmed theories about the crash," he wrote.
UIA said the plane underwent its last technical maintenance on January 6. It also said all flights to Tehran have been suspended indefinitely. The company said in a statement that most passengers were in transit and due to connect to other flights on arrival in Ukraine.
The plane's two black boxes had been retrieved and are being investigated by Iranian aviation experts, the semiofficial ISNA news agency reported.
The public prosecutor's office in Tehran said the remains of all passengers had been brought to forensics experts.
Airline manufacturers usually assist in crash investigations, but such an effort could be affected in this case by U.S. sanctions reinstated after President Donald Trump unilaterally withdrew from Iran's 2015 nuclear deal with world powers in May 2018.
Iran's semiofficial news agency Mehr quoted the head of the country's civil aviation authority as saying Iran would not give the black boxes to Boeing.
The Boeing 737-800 is a common single-aisle, twin-engine jetliner used for short- to medium-range flights.
Thousands of the planes are used by airlines around the world.
Introduced in the late 1990s, it is an older model than the Boeing 737 MAX, which has been grounded for nearly 10 months following two deadly crashes.
A number of 737-800 aircraft have been involved in deadly crashes recently.
A FlyDubai 737-800 from Dubai crashed in 2016 while trying to land at Rostov-on-Don airport in Russia, killing 62 on board.
Another 737-800 flight from Dubai, operated by Air India Express, crashed in 2010 while attempting to land in Mangalore, India, killing more than 150 on board.