A Kyiv-bound Ukraine International Airlines Boeing 737 plane crashed minutes after takeoff from Tehran, Iran, on Wednesday, killing all 176 people on board.
The Boeing 737-800, which was carrying 167 passengers and nine crew members, was an older model of the Boeing 737 — not a 737 Max, which has been grounded worldwide since mid-March after two fatal crashes.
Flight 752 crashed shortly after 6 a.m. local time, minutes into the flight. Press photos showed emergency workers combing through a wide field of smoldering wreckage outside Tehran.
The cause of the crash, which came hours after Iran launched retaliatory missile strikes on U.S. positions in Iraq for the killing of Gen. Qasem Soleimani, was not immediately known. Such determinations take months, but Iran's Fars news agency reported that the jet crashed due to technical problems, without providing more detail.
The timing of the crash prompted speculation that a stray Iranian missile may have downed the plane and aviation experts and pilots told CNBC that engine failure appeared unlikely.
Ukraine International Airlines said that the aircraft had been carrying citizens of Canada, Iran, Sweden and Ukraine. The carrier suspended flights to Tehran indefinitely. Ukraine's foreign minister separately said there were four passengers from Afghanistan, three from Germany, and three from Britain.
Ukraine International Airlines all but discounted pilot error and said the 3-year-old plane had been inspected Monday.
"Given the crew's experience, error probability is minimal," said Ihor Sosnovsky, vice president of operations. "We do not even consider such a chance."
The airline uses the airport "to conduct training on Boeing 737 aircraft aimed at evaluating pilots' proficiency and ability to act in emergency cases," he added.
The flight had an experienced crew: Capt. Volodymyr Gaponenko, who had 11,600 hours on the 737, 5,500 of them as captain, Ukraine International said. He was joined by first officer Serhii Khomenko who had 7,600 hours on a 737 and instructor pilot Oleksiy Naumkin, who had 12,000 hours on the 737, with 6,600 as captain, the airline said.
Under international law, the country where the crash took place leads the accident investigation but other nations often aid in the probe. Because the plane was a U.S.-made Boeing airliner, U.S. government officials and Boeing would normally be involved but tensions between the two countries called that into question. Iranian media reported that the country wouldn't provide the black box, which contains flight data, to the U.S. or Boeing.
It's unclear whether Iranian officials will send the black box to another country to be analyzed.
Ukraine's president, Volodymyr Zelenskiy, has also reportedly instructed the Ukrainian prosecutor general to open criminal proceedings over the crash, according to Reuters.
The 737-800 plane is not a 737 Max, the type that regulators grounded worldwide in March after two fatal crashes in a span of five months, sending Boeing into its biggest crisis in its more than 100-year history.
Ukraine International Airlines received the 737-800 jet in 2016 when it was new, according to Flightradar24, a flight-tracking site. Flight 752 stopped transmitting location data about two minutes into the flight, it added.
In a phone interview with the heads of the Civil Aviation and Emergency Management in Iran, state TV reported that the crash site was in Shahriar, near Tehran. Emergency services said the fire at the crash site was so intense that they had to briefly halt rescue efforts.
Ukraine International says it has 42 planes in its fleet and operates 1,110 flights a week. The airline did not immediately respond to a request for comment and more information.
"This is a tragic event and our heartfelt thoughts are with the crew, passengers, and their families," Boeing said in a statement. "We are in contact with our airline customer and stand by them in this difficult time. We are ready to assist in any way needed."
The engine-maker, CFM International, a joint venture of General Electric and Safran, said it was premature to speculate on the cause of the crash. Approximately 14,000 CFM56-7B engines are in service which have logged more than 400 million flight hours and the model is a staple of the Boeing 737NG fleet around the world.
"We are deeply saddened by the loss of Ukraine International Airlines Flight 752. We extend heartfelt sympathies to the families and loved ones of those on board," GE said in a statement.
The most fatal 737-800 crash was an Air India Express flight from Dubai in 2010 that killed 158 of the 166 people on board when it tried to land in Mangalore, India, according to the flight safety foundation. In 2016, a FlyDubai 737-800 crashed while trying to land in Rostov-on-Don Airport in southern Russia, killing 62 people on board.
— CNBC's Phil LeBeau contributed to this article.