Meanwhile, the probe into the crash was underway, with Iran pointing to a possible aircraft malfunction and Ukraine apparently leaving open other paths of inquiry.

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At least one U.S.-based aviation expert said it appeared the plane was "not intact" before it hit the ground. And a former Federal Aviation Administration accident investigation chief, Jeff Guzzetti, said the crash carried "all the earmarks of an intentional act."

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"I just know airplanes don't come apart like that," Guzzetti said.

The Ukraine International Airlines flight — bound for the Ukrainian capital, Kyiv — went down just before dawn after departing from Imam Khomeini International Airport, south of Tehran. The plane was approaching 8,000 feet when it abruptly lost contact with ground control, officials said.

About four hours earlier, Iranian forces launched more than a dozen ballistic missiles into Iraq, targeting an Iraqi air base with U.S. personnel and a facility in the northern city of Irbil in response to an American airstrike last week that killed the commander of Iran's elite Quds Force, Maj. Gen. Qasem Soleimani.

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American passenger airliners and others have avoided flying over Iran because of the risk that they could be mistaken for military aircraft. Iranian authorities said "technical" problems were probably behind the crash of the Ukrainian Boeing 737-800.

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Ukraine's embassy in Tehran initially concurred, issuing a statement ruling out terrorism and suggesting likely engine failure. It later took down the statement without explanation, raising questions about whether different scenarios — including an external cause such as a missile — were being explored as potential reasons for the crash.

But Iranian officials pushed back against that theory. Gen. Abolfazl Shekarchi, an Iranian armed forces spokesman, said "rumors" that a missile brought down the plane were "completely false."

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The Ukrainian Embassy said that a commission was investigating the crash and that "any statements about the causes of the accident before the decision of the commission are not official."

He was quoted by Iran’s Fars News Agency as calling the missile speculation “psychological warfare” by the government’s opponents.

A video circulated on Twitter that purported to be of the crash and showed the plane as a bright light, possibly on fire, descending against a dark sky, followed by a burst of flames.

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Guzzetti, the retired head of the FAA’s accident investigation division, said the publicly available details of the crash suggested the plane was brought down deliberately.

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“To me, it has all the earmarks of an intentional act. I don’t know whether it was a bomb or a missile or an incendiary device,” he said.

If the video of the flaming plane is accurate, “I can’t conceive of a failure that could cause that much of a conflagration,” he said.

An engine fire, for example, would take a substantial period “to consume the airplane,” said Guzzetti, who was an air safety investigator and engineering specialist at the National Transportation Safety Board for 18 years before joining the FAA.

The abrupt cutoff of flight-tracking data emitting from the plane also indicated that it was “a sudden catastrophic event that created a power loss throughout the whole airplane,” he said.

Todd Curtis, an aviation safety analyst for the website AirSafe.com and a former Boeing safety engineer who assisted in accident investigations, said it appeared — based only on video and photos from the crash site — that the plane was coming apart before it hit the ground.

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“The wreckage pattern was very consistent with a plane that was not intact when it hit the ground,” he said. “I didn’t see a large central crater.”

This means the crash could have been caused by an in-flight breakup, in-flight explosion, midair collision, structural failure, external strike or major system malfunction with the aircraft, Curtis said.

The head of Iran’s Civil Aviation Organization, Ali Abedzadeh, said that Iran would not send the recorders to the United States and that the investigation would be led by Iran, Mehr News Agency reported.

“It has not yet been decided where the [recorders] will go” for data extraction, Abedzadeh said.

He said the pilots “did not contact the control tower” before the crash. “We were not informed of any technical problem from the flight crew.”

Ukraine International Airlines purchased the plane as new, and its first commercial flight was June 2016, the carrier said. It had its last routine technical maintenance Monday.

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The possibility of “a mistake made by the crew is minimal; we just don’t assume that,” Ihor Sosnovsky, the airline’s vice president for flight operations, told a briefing, according to Interfax news agency. “Given their experience, it is very difficult to say that the crew may have done something wrong.”

Many of the passengers were Iranian. Ukraine’s Foreign Ministry listed 176 victims, including 82 Iranians; 63 Canadians; 11 Ukrainians, including nine crew members; 10 Swedes; four Afghans; three Germans; and three Britons. Iranian officials said more than 140 passengers were Iranians, suggesting that some may have had dual citizenship.

The Sharif University of Technology in Tehran said 13 of its students and alumni, most of them mechanical engineers or science students, perished in the crash.

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Some of the others on board included a newlywed couple, several young children and students attending universities in Canada and making their way back via Kyiv after visiting relatives in Iran during the winter break.

In Ottawa, Prime Minister Justin Trudeau said at least 138 people on board were booked to travel on from Kyiv to Canada.

Canadian officials would not publicly speculate on the cause of the crash, but Trudeau called for a full international investigation.

“There is a clear need for answers,” he told reporters.

Ukrainian President Volodymyr Zelensky said on his Facebook page that he was “personally involved in supervision over all measures to be taken.” A Ukrainian team planned to head to Iran to investigate the crash and identify the bodies of Ukrainians killed.

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In a briefing Wednesday, Ukrainian Prime Minister Oleksiy Honcharuk called for avoiding speculation about the cause of the crash and said Ukraine will insist on maximum transparency in the investigation.

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He said the government suspended all Ukrainian flights over Iranian airspace until “the reasons of the tragedy are determined.”

U.S. Secretary of State Mike Pompeo offered “all possible assistance” to Ukraine. In an apparent message to Iran, he said the United States “calls for complete cooperation with any investigation into the cause of the crash.”

Iran’s Civil Aviation Organization said that both the flight data recorder and the cockpit voice recorder were recovered but that the latter was severely damaged.

“Although it is damaged, there is still a chance to retrieve the pilots’ conversations from it,” a spokesman for the organization, Hassan Rezaeifar, told state media.

The Boeing 737-800 is a single-aisle aircraft designed for short- and medium-range flights. Airlines around the world have flown them for more than two decades, with thousands of them in service.

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But the plane has been involved in other incidents. The 2016 crash of a flight from Dubai killed 62, and a 2010 Ethiopian Airlines crash near Beirut claimed 90 lives.

Regulators have more recently scrutinized possible safety risks on the 737-800. In early October, the FAA told airlines to inspect more than 1,900 Boeing jets after cracks were found in some of the aircraft’s wings. Dozens of them were later grounded after cracks were found in a part of the plane that connects the wings to the fuselage.

The newer 737 Max was grounded worldwide last year after the crash of an Ethiopian Airlines flight on March 10, the second fatal crash of a 737 Max in less than five months. In October 2018, a Lion Air flight crashed off the coast of Indonesia, killing all aboard.

The grounding resulted in a crisis for Boeing that led to its firing of CEO Dennis Muilenburg two weeks ago.

“This is a tragic event and our heartfelt thoughts are with the crew, passengers and their families,” a statement from Boeing said. “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.”

Major international airlines — including the flagship carriers of France, Canada, Germany and the Netherlands — also either halted flights to Iraq and Iran or restricted aircraft from flying through both countries’ airspace. Germany’s Lufthansa said it would resume Tehran flights Thursday.

Other commercial airlines also rerouted flights, including Australian carrier Qantas, Malaysia Airlines and Singapore Airlines.

On Wednesday, Canada updated its travel advisory for Iran, warning against any nonessential travel to the country “due to the volatile security situation, the regional threat of terrorism and the risk of arbitrary detention.” Ukraine International Airlines lists its Tehran-to-Toronto service as one of its most popular routes.

Correction: A previous version of this report incorrectly stated the death toll from a 2018 crash in Papua New Guinea that involved a Boeing 737-800. One person, not 47, died.