MOSCOW — Western leaders sketched out Thursday what they say was the fate of Ukraine International Airlines Flight 752 — apparently downed by an Iranian missile, possibly fired by mistake, as the plane with 176 people aboard climbed above Tehran's outskirts.

The assessments — based on intelligence reports from the United States, Canada and elsewhere — were met with quick dismissals from Iran and suggested deepening divides over the unfolding investigation of Wednesday’s crash, which killed all aboard.

In Ottawa, Canadian Prime Minister Justin Trudeau said intelligence analyses indicated that a surface-to-air missile hit the plane in what could have been an “unintentional” act.

In Washington, U.S. officials expressed “high confidence” that the Boeing 737-800, bound for the Ukrainian capital of Kyiv, was targeted by air-defense systems as Iranian forces were on high alert. President Trump said the downing of the aircraft was probably a “mistake.”

Trudeau’s comments — as his nation mourned 63 Canadians on the flight and dozens of others with links to the country — came after U.S. officials said the plane was apparently hit by an SA-15 surface-to-air missile, part of a Russian-made air defense system also known as a Tor system.

The U.S. officials, speaking on the condition of anonymity to discuss intelligence matters, offered no other details on the circumstances that led to the possible missile firing Wednesday, which came about four hours after Iran launched ballistic missiles into Iraq against U.S. targets in retaliation for the killing of Iranian military commander Qasem Soleimani.

Iran has repeatedly rejected theories of a missile strike. A military spokesman on Wednesday called it “ridiculous.” Iran’s state-run Islamic Republic News Agency labeled the claims Thursday a “psychological operation” waged by the Pentagon.

As Western officials built their case, it was unclear whether Iran would publicly acknowledge the direction of the probe or offer further details on the minutes after the flight left the runway, rose toward 8,000 feet, then burst into flames without a distress call.

And with the region on edge, Iranian military leaders could be put in a difficult spot, with commanders left to face possible questions about procedures or even the locations of antiaircraft sites.

Trudeau, speaking to reporters, said the missile-strike conclusion was based on “intelligence from multiple sources, including our allies and our own intelligence.”

“The evidence indicates the plane was shot down by an Iranian surface-to-air missile. This may well have been unintentional,” he said, without giving further details.

Trump, speaking Thursday at the White House, also said the passenger jet could have been downed in error. “Well, I have my suspicions,” he said. “I don’t want to say that, because other people have those suspicions also. . . . Somebody could’ve made a mistake on the other side.”

“Some people say it was mechanical,” Trump added. “Personally, I don’t think that’s even a question.”

In London, British Prime Minister Boris Johnson joined the growing consensus around the missile strike. He cited a “body of information that the flight was shot down by an Iranian surface-to-air missile.”

The Washington Post obtained a video that allegedly shows the moment the airliner is struck in midair. The video, first published by the New York Times, purportedly shows a missile intercepting the aircraft, followed by a loud boom.

Flight tracking data from Flightradar24 showed the Ukrainian airliner taking off from Tehran Imam Khomeini International Airport in a northwesterly direction before losing contact near the suburb of Parand.

Michael Elleman, director of the Non-proliferation and Nuclear Policy Program at the International Institute for Strategic Studies, said the general area about 12 miles northwest of Tehran’s airport is known to be filled with Iranian missile facilities, which could have been protected by antiaircraft systems. The Iranian government may have suspected the missile sites could be targets for a retaliation by the United States.

The Tor system dates to the Cold War and can target planes, helicopters, drones or incoming missiles. Russia has exported the system to a number of countries, including Iran in 2005. It is designed to hit targets in the short-to-medium range.

Ukrainian investigators

A team of 45 experts and search-and-rescue personnel from Ukraine arrived in Tehran early Thursday to participate in the probe, as well as to identify and repatriate the bodies of the 11 Ukrainians aboard, including all nine crew members.

Before the U.S. and Canadian statements, Ukrainian investigators had said they were also looking at a range of theories — including the possibility of antiaircraft fire.

Oleksiy Danilov, secretary of Ukraine’s National Security and Defense Council, wrote on Facebook that his team wanted to search for possible debris from a Tor air-defense missile, after seeing online reports about the discovery of possible fragments of one near the crash site.

Ukraine is familiar with the risks that antiaircraft weapons can pose to civilian aircraft in times of conflict.

In July 2014, Malaysia Airlines Flight 17 was downed by a missile shot from a Russian-made Buk surface-to-air missile system from rebel territory in eastern Ukraine. The attack on the Boeing 777, which was passing over the conflict zone while flying from Amsterdam to Kuala Lumpur, killed all 298 people aboard.

A joint investigative team from Australia, Belgium, Malaysia, the Netherlands and Ukraine identified a Russian military unit in charge of the antiaircraft missile system and has pursued prosecution of Russian and Ukrainian citizens allegedly involved. Russia has denied any involvement in the incident.

Danilov said that Ukraine’s investigative commission for the Tehran crash includes specialists who helped investigate the Flight 17 crash site. The government in Kyiv has also suspended all Ukrainian flights over Iranian and Iraqi airspace.

Iran's air-defense structure

The responsibility for air defense traditionally falls to Iran’s Air Defense Force command, said Saeid Golkar, a political science professor at the University of Tennessee at Chattanooga who has studied Iranian security forces for 20 years.

Orders to mobilize the country’s air defense are given by a central command center, which includes air-defense batteries controlled by Iran’s two armed forces: the regular military and the Islamic Revolutionary Guard Corps. Within this system, the Revolutionary Guard is also in control of its own air-defense systems and has the capability of acting without authorization, Golkar said.

“It is possible that the IRGC acted alone?” Golkar asked.

“If you asked me if its possible if someone mistakenly shot it down, I’d say yes, because of the whole informality of the system,” Golkar said.

A preliminary report from Iran’s Civil Aviation Organization said witnesses — on the ground and among the crew of another flight in the vicinity — reported seeing a fire while the Boeing 737-800 was still in the air, followed by an explosion when it slammed into a field near an amusement park.

A video from a closed-circuit television camera posted on Twitter by Iran’s state broadcaster showed the predawn darkness suddenly lit in a fiery orange glow and then flaming debris scattering over a wide area.

“The trajectory of the collision indicated that the plane was initially moving toward the west, but after encountering a problem, it turned to the right and was approaching the airport again at the time of the crash,” Ali Abedzadeh, head of the Civil Aviation Organization, said in the report.

Iranian officials said immediately after the crash that the plane had encountered technical problems, but that did not appear in the report, which also noted that there was no distress call from the aircraft.

The passengers on the plane were mostly Iranians but also included Europeans and Canadians.

The Iranian report said the flight data recorder and cockpit voice recorder were recovered but were damaged. Abedzadeh has said Iran will not share those “black boxes” with Boeing, but other countries have been invited to participate in the investigation in accordance with international guidelines.

John Cox, an airline safety consultant and former pilot, said Iranian officials have indicated they will conduct their probe in accordance with International Civil Aviation Organization Annex 13, which outlines the process by which international crashes are investigated. Under the protocols, the United States has a right to participate in the probe because the jet was built and certified in the country.

The U.S. National Transportation Safety Board on Thursday declined to say whether it will participate in the investigation. However, two people with knowledge of the investigation but not authorized to speak publicly said the safety body had been asked by the ICAO to take part in the probe.

Still, U.S. sanctions against Iran, which include limits on travel and information-sharing, will complicate any U.S. involvement.

NTSB officials would have to seek a license from the U.S. Office of Foreign Assets Control to travel to Iran, a process can take months or even years, experts say. And even if they secure a license, U.S. officials may still be limited in what information they can share with Iranian officials.

Canada cut diplomatic ties with Iran about a decade ago. Trudeau said that while there had been “openness” from Iranian officials to involve Canada in the investigation, access had not yet been given to them. Iran wanted to keep the black boxes there but told Ukrainian President Volodymyr Zelensky that Ukrainian investigators would have access to them, Trudeau said.

At his news conference, Trudeau dodged questions about whether the United States should bear some responsibility since its killing of Soleimani last week escalated tensions in the region.

“I think that’s one of the many questions that people will be thinking about and trying to find answers to,” he said. “But for the moment, I just want to underline the importance of having a full and credible investigation so that we can get those facts, and then we can continue to analyze based on these facts.”

Lamothe, Ryan and Sonne reported from Washington. Amanda Coletta in Toronto, Erin Cunningham in Istanbul and Lori Aratani, Dalton Bennett, Shane Harris, Michael Laris and Souad Mekhennet in Washington contributed to this report.