A Western intelligence official told NBC News that there was high confidence that the plane was felled by an accidental missile strike. The source, who receives high-level briefings from U.S. intelligence agencies, said the evidence includes satellite imagery and communications intercepts.
"Not much goes on above head height in Iran without it being tracked," the source said.
Prime Minister Justin Trudeau said Canadian intelligence sources also pinned blame on Iran for what might have been an "unintentional" missile attack.
"We have intelligence from multiple sources, including our allies and our own sources. The evidence indicates that the plane was shot down by an Iranian surface-to-air missile," Trudeau told reporters in Ottawa. "This may well have been unintentional."
Trudeau sidestepped several questions asking whether Canada should blame the United States for its deadly attack on a top Iranian military leader, which has renewed tension between Tehran and Washington.
"I think that's one of the many questions that people will be thinking about and trying to find answers to," Trudeau said in French. "But for the moment, I just want to underline the importance of having a full and credible investigation so we can get those facts and we can continue to analyze based on these facts."
British Prime Minister Boris Johnson released a statement saying his thoughts were with the families of the four British nationals who were killed, and he echoed Trudeau's assertion that an Iranian mistake might have been responsible.
"There is now a body of information that the flight was shot down by an Iranian Surface to Air Missile," Johnson said. "This may well have been unintentional. We are working closely with Canada and our international partners and there now needs to be a full, transparent investigation."
An initial Iranian report released Thursday suggested that a sudden emergency struck the Boeing 737 before it went down just moments after having taken off from Imam Khomeini International Airport in Tehran. The report said the crew never made a radio call for help and were trying to turn back for the airport when the plane crashed.
The Boeing 737 has several systems to announce itself to other aircraft and to ground-based air-traffic control. The 737 includes a state-of-the-art Automatic Dependent Surveillance-Broadcast, or ADS-B, system, which automatically transmits altitude, airspeed and location and generally indicates that an aircraft is not part of a military operation.
Tor missile systems are designed to be mobile and self-contained, meaning they can rely on their own radar system to detect targets, rather than connect to a larger integrated air-defense system that monitors civilian air traffic.
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Flight 752 lifted off at the end of a tense week in which missile batteries were repositioned throughout Iran, possibly in anticipation of a U.S. attack. Anti-aircraft batteries are often the first things to be destroyed in an attack, placing the crews inside them under enormous psychological pressure. Radar crews are trained to use their equipment only briefly, as radar is easy to detect and target.
While the takeoff and steady climb of a passenger plane are obvious to an experienced crew watching it for several minutes on radar, and while even a civilian phone app like FlightAware could have identified the plane as a civilian aircraft from its ADS-B system, a crew on the ground outside Tehran may not have had enough radar time or access to civilian information systems to identify it accurately.
Investigators from Iran's Civil Aviation Organization have offered no immediate explanation.
President Donald Trump, asked Thursday for his thoughts on what happened, said, "Well, I have my suspicions."
"It's a tragic thing," Trump said. "Because somebody could've made a mistake on the other side. ... It was flying in a pretty rough neighborhood, and somebody could, could've made a mistake."
The plane was carrying 167 passengers and nine crew members from several countries, including 82 from Iran, at least 63 from Canada and 11 from Ukraine, according to officials.
Many of the passengers were believed to have been international students attending universities in Canada; they were making their way back to Toronto by way of Kyiv after having visited their families during the winter break.
The flight also carried newlyweds and a family of four. The passenger list included several teenagers and children, some as young as 1 or 2.
The jet crashed hours after Iran fired missiles at two Iraqi bases housing U.S. forces early Wednesday local time.
The missile strikes were Iran's retaliation for the U.S. drone attack that killed Iranian Maj. Gen. Qassem Soleimani on Jan. 3. U.S. authorities later reported that there were no casualties from the Iranian attack on the air bases.
Ukrainian President Volodymyr Zelenskiy pledged to discover the truth and announced that investigators from his country had arrived in Iran.
The Ukrainian Embassy in Iran originally said that the crash likely was caused by an engine problem, ruling out terrorism or a rocket attack. However, it later removed that information from its website and said the cause was under investigation.
Witnesses, including the crew of passing flight, described seeing the plane engulfed in flames before it went down, the Iranian report said.
The crash caused a massive explosion, likely because the aircraft had been fully loaded with fuel for the flight to Kyiv, Ukraine. It scattered flaming debris and passengers' belongings across a wide stretch of farmland.
Both of the so-called black boxes recording data and cockpit communications were recovered, although they were damaged, the report said.
Zelenskiy said he planned to call Iranian President Hassan Rouhani about the crash and the investigation.
"Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash," he said. "We will surely find out the truth."
Zelenskiy said the crash would be investigated by a committee created by Iran's civil aviation agency. He cautioned against speculation and conspiracy theories while the investigation was ongoing.
"I call on the international community, including Canada, to join the investigation," he added.
Even though Ottawa has no diplomatic ties to Tehran, Trudeau demanded that Canadian investigators be part of the investigation.
"We have already been engaged with the Ukrainians who are part of the investigation team," Trudeau said. "And Iran has indicated an openness to Canada being engaged, as well. To which degree and in what way remains to be worked out. But there is an openness there."
Ukrainian officials initially agreed with Iranian suspicions that the 3½-year-old plane was brought down by mechanical trouble, but they later backed away and declined to offer a cause while the investigation was ongoing.
A top Ukrainian security official said in a Facebook post Thursday that four main scenarios were are being looked into: an anti-aircraft missile strike, a collision with an unmanned aerial vehicle, an engine failure and an explosion inside the aircraft as the result of a terrorist act.
Thursday was declared a national day of mourning in Ukraine.
The disaster could further damage Boeing Co.'s reputation, which has been battered by the furor over two deadly crashes involving a different Boeing model, the newer 737 Max, which has been grounded for nearly 10 months.